Screening of Oxygen-Carrier Particles Based on Iron-, Manganese-, Copper- and Nickel Oxides for Use in Chemical-Looping Technologies

THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Screening of oxygen-carrier particles based on iron-, manganese-, copper- and nickel oxides for use in chemical-looping technologies

MARCUS JOHANSSON

Department of Chemical and Biological Engineering Environmental Inorganic Chemistry Chalmers University of Technology Göteborg, Sweden 2007

Screening of oxygen-carrier particles based on iron-, manganese-, copper- and nickel oxides for use in chemical-looping technologies

MARCUS JOHANSSON ISBN: 978-91-7385-037-7

Doktorsavhandlingar vid Chalmers tekniska högskola Ny serie nr 2718 ISSN: 0346-718X

Department of Chemical and Biological Engineering Environmental Inorganic Chemistry Chalmers University of Technology

SE-412 96 Göteborg Sweden

Telephone + 46 (0)31-772 1000

[email protected]

Cover: A scanning electron microscopy image of a particle consisting of 40% NiO with 60% of MgAl2O4 sintered at 1400 °C.

Cover printed by: Chalmersbibliotekets reproservice Göteborg, Sweden 2007

II Screening of oxygen-carrier particles based on iron-, manganese-, copper- and nickel oxides for use chemical-looping technologies

Marcus Johansson

Department of Chemical and Biological Engineering Environmental Inorganic Chemistry, Chalmers University of Technology

ABSTRACT

Capture and storage of carbon dioxide from combustion will likely be used in the future as a method of reducing emissions of greenhouse gases and thus be part of the overall strategy to stabilize the atmospheric levels of CO2. Chemical-looping combustion is a method of combustion where CO2 is inherently separated from the non-condensable components in the flue gas without the need for an energy intensive air separation unit. This is because nitrogen from the combustion air is never mixed with the fuel. Instead, oxygen carriers, in the form of metal oxide particles, circulate between two interconnected fluidized reactors and transfer oxygen from the air to the fuel through heterogeneous gas-solid redox reactions. The technology could also be adapted for the production of hydrogen from fossil fuels with CO2 separation, i.e. chemical-looping reforming.

108 different oxygen-carriers based on iron-, manganese-, copper- and nickel oxides have been investigated. These carriers are prepared with inert material to increase the lifetime and performance of the particles. All particles but one have been produced by a freeze-granulation method. In order to optimize the performance of the particles, the sintering temperature of the particles was varied between 950°C and 1600°C. Normally particles of the size range of 125-180 μm have been used for the reactivity investigations. Screening tests were performed in a laboratory fluidized-bed reactor of quartz placed in a furnace. The particles were exposed to an environment simulating a real chemical- looping combustor, by alternating between reducing (50% CH4 / 50 % H2O) and oxidizing conditions (5% O2 in N2). The temperature was varied in the range 600 – 950°C with most experiments conducted at 950°C. In addition the particles were characterized with respect to strength, physical appearance and chemical structure before and after the experiments. Some suitable oxygen carriers were investigated in more detail in the fluidized bed, and parameters such as reaction temperature, particle size, reducing gas and experimental method were varied.

With respect to reactivity with methane, the different oxygen carriers can generally be ranked in the order nickel> copper> manganese> iron whereas the crushing strength roughly follows the opposite order. Several types of inert material were used in this work, and this was found to be a very important parameter. It was found that inert material based on alumina and zirconia in general resulted in promising oxygen carriers, whereas titania, silica and magnesia were less promising with respect to reactivity or lifetime of the particles. Using a low sintering temperature in preparation is associated with a high reactivity, but also a low strength. This is because the higher temperatures provoke a breakdown of the internal porous structure which also makes them denser. Twelve out of the initial 108 particles were not useful for different reasons, including melting, lack of structure and lack of reactivity due to formation of non-reducible species.

The majority of the investigated oxygen carriers are well suited for chemical-looping combustion taking into consideration the important criteria of reactivity, high conversion of the fuel, relatively high strength and ability to withstand de-fluidization, agglomeration, fragmentation and abrasion.

Keywords: Chemical-Looping Combustion, Chemical-Looping Reforming, Oxygen Carrier, Carbon Dioxide Capture, Nickel Oxide, Copper Oxide, Manganese Oxide, Iron Oxide.

III

LIST OF PUBLICATIONS

This thesis is based on the work contained in the following papers, referred to by Roman numbers in the text:

I Johansson M, Mattisson T, Lyngfelt A, 2006, Comparison of Oxygen Carriers for Chemical-Looping Combustion. Thermal Science, 10, (3), 93-107.

II Johansson M, Mattisson T, Lyngfelt A, 2006, Comparison of Oxygen Carriers for Chemical-Looping Combustion of methane-rich fuels. 19th FBC Conference, May 21-24, Vienna

III Mattisson T, Johansson M, Lyngfelt A, 2004, Multi-Cycle Reduction and Oxidation of Different Types of Iron Oxide Particles. Energy & Fuels, 18, (3), 628-637.

IV Mattisson T, Johansson M, Lyngfelt A, 2006, The use of NiO as an Oxygen Carrier in Chemical- Looping Combustion. Fuel, 85, (5-6), 736-747.

V Johansson M, Mattisson T, Lyngfelt A, 2006, Investigation of Mn3O4 with stabilized ZrO2 for Chemical-Looping Combustion. Chemical Engineering Research and Design, 84, (A9), 807-818.

VI Johansson M, Mattisson T, Lyngfelt A, 2004, Investigation of Fe2O3 with MgAl2O4 for Chemical- Looping Combustion, Industrial and Engineering Chemistry Research, 43, (22), 6978-6987.

VII Johansson M, Mattisson T, Lyngfelt A, 2006, Use of NiO/NiAl2O4 particles in a 10 kW Chemical- looping Combustor, Industrial and Engineering Chemistry Research, 45, (17), 5911-5919

VIII Mattisson T, Johansson M, Jerndal E, Lyngfelt A, 2007, The reaction of NiO/NiAl2O4 particles with alternating methane and oxygen, Accepted for publication in Canadian Journal of Chemical Engineering

IX Mattisson T, Johansson M, Lyngfelt A, 2006, CO2 capture from coal combustion using chemical-looping combustion – Reactivity investigation of Fe, Ni and Mn based oxygen carriers using syngas, Clearwater Coal conference, Clearwater, FL

IV X Johansson M, Mattisson T, Lyngfelt A, 2006, Creating a Synergy effect by using mixed oxides of iron- and nickel oxides in the combustion of methane in a chemical-looping combustion reactor. Energy & Fuels, 20, (6), 2399-2407.

XI Johansson M, Mattisson T, Lyngfelt A, Abad, A, 2007, Using continuous and pulse experiments to compare two promising nickel-based oxygen-carriers for use in chemical-looping technologies Fuel, available on-line 11 September 2007

Papers not included in this thesis: Johansson M, Mattisson T, Lyngfelt A, 2005, Comparison of Oxygen Carriers for Chemical-Looping Combustion. International Symposium "Moving Towards Zero-Emission Plants", Leptokarya Pieria, Greece, June 20th-22nd (Shorter version of Paper I)

Mattisson T, Zafar Q, Johansson M, Lyngfelt A, 2006, Chemical-looping combustion as a new CO2 management technology, First Regional Symposium on Carbon Management, Dhahran, Saudi- Arabia, May 22-24

Johansson M, Mattisson T, Rydén M, Lyngfelt A, 2006, Carbon Capture via Chemical-Looping Combustion and Reforming, International Seminar on Carbon Sequestration and Climate Change, Rio de Janeiro, Brazil, 24-27 Oct.

Abad, A, Mattisson, T, Lyngfelt, A, Johansson, M, 2007, The Use of Iron Oxide as Oxygen Carrier in a Chemical-Looping Reactor, Fuel 86, (7-8), 1021-1035

My contribution to the papers included in this thesis: Papers I, II, V, VI, VII, X & XI; all experimental work, all evaluations and writing Papers III, IV & IX; all experimental work and evaluation of experiments Paper VIII; experimental work performed in batch-fluidized reactor and evaluation of this data

1. Introduction ...... 1 1.1. Greenhouse gases...... 1

1.2. Storing of CO2...... 3 1.2.1. Storing facilities ...... 3 1.2.2. Transportation, legal aspects and public acceptance of storing...... 8 1.3. Sequestration techniques ...... 9 1.4. Chemical-Looping Combustion (CLC)...... 12 1.4.1. Integration with power process and thermal efficiencies...... 15 1.4.2. Chemical-Looping Reforming (CLR)...... 16 1.4.3. Chemical-Looping Combustion of solid fuels...... 18 1.4.4. Experiments of chemical-looping combustion and reforming in prototype units19 1.4.5. Oxygen Carriers...... 20 1.4.6. Oxygen Carriers in literature...... 24 1.4.7. Oxygen Carriers in this study...... 30 1.5. Objective...... 35 2. Experimental...... 37 2.1. Method and purpose of investigation ...... 37 2.2. Preparation of oxygen carriers ...... 38 2.3. Characterization of fresh and reacted oxygen carriers ...... 39 2.4. Reactivity Investigation...... 39 2.4.1. Pulse experiments...... 48 3. Results and discussions...... 51 3.1. The influence of the sintering temperature ...... 51 3.2. Results from different metal oxides using methane as fuel...... 55 3.2.1. Iron based oxygen carriers...... 55 3.2.2. Manganese based oxygen carriers ...... 56 3.2.3. Copper based oxygen carriers ...... 57 3.2.4. Nickel based oxygen carriers...... 58 3.2.5. Mixed oxide systems...... 60 3.3. Results from different inert material using methane as fuel...... 62

3.3.1. Al2O3 based inert material...... 62

3.3.2. ZrO2 based inert material ...... 63

3.3.3. TiO2 as inert material...... 64

3.3.4. SiO2 and MgO as inert material ...... 64 3.4. Comparison of the oxygen carriers ...... 65 3.5. Results from experiments using syngas as fuel...... 67 3.6. Results from pulse experiments using methane as fuel...... 69 3.7. De-fluidization ...... 72 3.8. Carbon Formation...... 75 3.9. Kinetics ...... 76 4. Conclusions...... 79 5. Notations ...... 81 6. Acknowledgement...... 83 7. Appendix – XRD of investigated oxygen carriers ...... 85 8. Appendix – SEM images of investigated oxygen carriers ...... 87 9. References...... 93

1. Introduction

1.1. Greenhouse gases

For more than a hundred years it has been known that carbon dioxide, CO2, is a greenhouse gas that contributes to the warming of the earth. CO2 is a naturally occurring gas that helps to keep the temperature agreeable on earth. This is because carbon dioxide, together with other greenhouse gases, acts as a protecting layer in the troposphere that prevents most of the outgoing long-wave radiation from the earth from leaving the atmosphere. The problem that has been discussed in recent decades is that the amount of CO2 has risen to such high levels that a warming of earth, above normal values, is occurring. The concentration of CO2 in the atmosphere today is approximately 380 ppm, more than 30% higher than the pre-industrial level of 280 ppm.[1-3] The average global temperature increase of 0.74±0.18°C [3] that we have experienced during the last 100 years is most likely an effect of this higher level and its trend is shown in Figure 1-1.

Figure 1-1 The observed change in global mean temperature in the last 140 years and the temperature change in the Northern Hemisphere in the last 1000 years [4]

This temperature increase is believed to be associated with anthropogenic CO2 emissions mainly coming from combustion of fossil fuels. Due to this increase and the possible frightful climate changes it may result in [3, 5], the Kyoto Protocol was proposed. In this, the developed countries agreed to reduce their CO2 emissions to 5.2% below their 1990 levels in the period between

2008-2012.[6] It should be noted that CO2 is only one of six greenhouse gases mentioned in the

Kyoto Protocol. The five other gases are CH4, N2O, hydro-fluorocarbons (HFCs), perfluorocarbons (PFCs) and SF6.[7] Global emissions of the greenhouse gases have increased by * 70% between 1970 and 2004 expressed in carbon dioxide equivalents (CO2-eq) . Of these, CO2 contributes to about 77%.[7] Hence, the considerably higher content of CO2 in comparison to other greenhouse gases and the constant increase of its concentration in the atmosphere make it the focus of attention for solving the problem of excessive global warming.[8] Various climate models predict different future scenarios of the CO2 concentration in the atmosphere and its associated temperature rise. Due to these uncertainties, a definite number on an acceptable CO2 level in the atmosphere can not be defined. However, most scenarios suggest that we need to start decreasing the emissions of greenhouse gases, or at least keeping them at current levels, in order not to jeopardize our future. Hence we should aim at releasing less than the current emissions of 22-27 GTonnes CO2/year (7-8 GTonnes C/year).[1, 3, 8-10]

Since energy demand is closely associated to GDP (gross domestic product), it is very likely that the demand for energy will continue to increase in the future as an effect of economic growth, especially in the developing countries.[1, 7, 11, 12] In order to solve the difficult equation of meeting an increasing energy demand and at the same time decrease the CO2 emissions, three general routes are proposed;[8, 9, 12]

• Increase the efficiency of energy conversion and increase the efficiency in energy usage. • Reduce the carbon intensity in the energy supply. This includes change to fuels of less or no carbon content, e.g. nuclear energy, and use of renewable energy sources, such as biofuels, solar- and wind power.

• Sequestrate the CO2 produced. This means capture and storage of CO2 from combustion of fossil fuels. This option is often referred to as “Carbon capture and storage”, CCS.

* The definition of carbon dioxide equivalent (CO2-eq) is the amount of CO2 emission that would cause the same radiative forcing as an emitted amount of well mixed greenhouse gas or a mixture of well mixed greenhouse gases, all multiplied by their respective “global warming potential (GWP) to take into account the differing times they remain in the atmosphere.

The first two options are clearly in-line with sustainable development but may not be sufficient at present, due to our great dependence on fossil fuels. Therefore, capture and storage may be important for reducing CO2 emissions to the atmosphere sufficiently fast.[7, 9, 12]

About a third of the global anthropogenic CO2 emissions come from the burning of fossil fuels in power production and roughly a fifth arises from the transportation sector.[8, 13] There are major structural differences between these sources of CO2. In the transport sector the emissions are divided into numerous mobile sources and each of these releases a relatively small amount of

CO2. It is not feasible to capture CO2 from each vehicle and hence, to reduce emissions from the transport sector a change to renewable fuels, hydrogen or electricity is necessary. To avoid net emissions of CO2, hydrogen and electricity as “fuel” for vehicles needs to be produced from non fossil energy sources or at large fossil fuel plants where the CO2 could be captured and stored.[11] Similarly, in the domestic sector it does not seem to be technically and economically feasible to install capture technology on small heating units. As with the transportation sector, a change in energy supply (e.g. to district heating or heat pumps) could improve the possibilities of limiting the CO2 emissions. For the sector of power production the conditions are quite the opposite and therefore a change of fuel does not necessarily need to take place. The sources are stationary and usually very large, which facilitates collection and transport of CO2. For energy intensive industry, such as ammonia production, oil refining, cement or gas processing, a way to sequestrate and limit CO2 emissions could also be useful.[2, 10, 11]

1.2. Storing of CO2

1.2.1. Storing facilities

Known resources of fossil fuel, i.e. both conventional and non-conventional, are estimated to contain carbon that is equivalent to emissions of about 20 000-25 000 GTonnes of CO2, if burned.[5, 14] When stored, it is of greatest importance that no CO2 leaks into the atmosphere for a very long time period. The needed storage time can be estimated to be in the order of 1000 - 10 000 years.[1, 14] Hence, space and time requirements put a lot of demands on a possible storage location. Furthermore it is important that the storage will not affect the environment negatively. Several different options for storing of CO2 have been discussed; these include storage

3 in deep ocean, storage in oil fields (also called enhanced oil recovery, EOR), storage in gas fields, storage in deep coal beds (ECBM), and storage in aquifers. These options are summarized in Figure 1-2.

Figure 1-2 Options for storage of CO2 [6]

The capacity for deep ocean storage is enormous and no real physical limit can be defined.[12] Various ideas for performing forced (as opposed to natural) storage in oceans have been proposed. One example is storing of CO2 in a deep ocean bed lake at depths below 3000 m, where CO2 has a higher density than sea water.[5, 8, 15] However, the environmental and legal aspects of ocean storage are controversial and uncertain, and therefore this option is not one of the main alternatives for storing of CO2. [1, 5, 8, 16, 17]

Injection in oil fields is performed today although the purpose is not to reduce CO2 emissions to the atmosphere. Instead the technique is used to increase the yield and efficiency of recovering oil since CO2 acts as an agent to force oil out of the almost depleted fields. Enhanced oil recovery, EOR, started first 1972 and today 33 MTonnes CO2/year is used in more than 70 projects in USA.[1, 6, 11, 18] In the Weyburn project in Canada, EOR is for the first time used

4 with only anthropogenically produced CO2, and here the fate of the stored gas is monitored.[18, 19] The advantage with EOR is that the cost is negative, i.e. the value of the extracted fuel is larger than the storage cost, and furthermore the geology of the fields is well known. Disadvantages include the limited storage volume.[6, 12]

Storing of carbon dioxide could be realized in depleted gas fields. The geology of these fields is well known and have by definition shown that they are able to hold gas for millions of years.[20] An example of such storing is the on-shore In-Salah project in Algeria where approximately 1.2 million tons of CO2 has been stored annually in an depleted gas field since the start in 2004.[21] Another possibility is enhanced gas recovery (EGR) which could be used to increase the yield of an almost depleted gas field, i.e. similar to EOR.

Storing in unminable coal is quite similar to storing in gas and oil fields. In unminable coal beds methane is trapped and can be extracted by injecting CO2. About twice the volume of CO2 can be stored as compared to CH4 extracted. The disadvantage of this option is the comparably very low storage capacity, estimated to be less than 15 GTonnes CO2.[6, 12]

For storing in aquifers the capacities could be enormous, somewhere between 400 and 10 000

GTonnes CO2.[6, 12] The storage in these underground permeable rock formations takes place in the pores of sandstone and limestone, formed through accumulation of sand, clay and organic material. These sedimentary rocks are options for storage if CO2 can be trapped by an overlaying non-porous rock, so called cap rock.[1, 2, 11, 20] In the pores the CO2 will act as a liquid and either be mixed with saltwater or react with minerals to carbonates.[1, 20]

Currently there is a major CO2 storage project in an aquifer at the Sleipner natural gas field in The

North Sea. Here natural gas with a high content of CO2 is upgraded to a purer product when the

CO2 concentration in the gas is decreased from 9% to 2.5%.[18, 19] The separated stream of CO2 is injected 800 m below the seabed in the Utsira formation which can be seen in Figure 1-3. Since

1996, approximately 1 MTonnes of CO2 has been injected annually.[22, 23] The CO2 project in the Sleipner field was the first time ever that CO2 was stored for environmental reasons.

Figure 1-3 CO2 injection into the Utsira deep salt-water reservoir [6]

A summary of the ongoing and proposed large-scale facilities for storing (and monitoring) of

CO2 is found in Table 1-1.

Table 1-1 Undergoing and proposed projects for large scale storing of CO2

Starting year Location CO2 injection (million tons/year) 1996 Sleipner, Norway 1-1.2 2000 Weyburn, Canada 1.1- 1.8 2004 In-Salah, Algeria 1.1 - 1.5 2004 K12B, Netherlands 0.2 – 0.4 2004 Frio, USA 0.65 2006 Snøhvit, Norway 0.7-1 2009 Gorgon, Australia 3.3-5 Data taken from[1, 12, 18]

There are also several other options for storing CO2 which are based on the natural ability of the biosphere to bind carbon; the oceans, the forest and rock weathering. The total amount of carbon in the oceans correspond to 140 000 GTonnes CO2 (38 000 GTonnes C).[5, 8] Eventually the oceans will be able to absorb major parts of the excessive CO2 in the atmosphere but this is

6 associated with too long time scales to provide any solution to the global warming.[5, 8] And besides, since dissolved CO2 lowers the pH of the oceans, the long term environmental effect of ocean storage is not known. Currently the oceans are annually absorbing about 6.2 ± 1.8

GTonnes CO2 (1.7 ± 0.5 GTonnes C).[8]

Cultivating new forest has also been proposed as one way of diminishing CO2 in the atmosphere due to the photosynthesis. The negative aspect of forest binding of CO2 is the limited amount of storage capacity and the insecurity; can a forest be considered as safe storage for thousands of years? Whether existing forests are already saturated with CO2 or if CO2 actually could stimulate growth of forest is however not fully understood.[24]

Rock weathering is a natural process where rocks consisting of magnesium- and calcium silicates react with CO2 from the atmosphere to form very stable carbonate rocks. The capacity for this kind of storage is huge, larger than all CO2 from the fossil fuel reserves, however the kinetics are extremely slow.[8, 12]

For different reasons mentioned these three natural storing options should not be seen as solutions to our current problem of needing fast but long-lasting storing CO2 from power production.

A summary of the geological storage options and their capacities are presented in Table 1-2. It should be mentioned that the actual figures are dependent on the source. For simplicity, values are taken from the IEA Greenhouse Gas R&D Program and the International Panel on Climate Change, IPCC.[6, 12] The major interest here is to compare the magnitude of different storage capacities to each other.

Table 1-2 The potential of geological storage options.

Geological Storage Option GTonnes CO2 Deep ocean storage No real limit Depleted Oil and Gas Fields 675-920 Unminable coal seams 3-15 Deep saline reservoirs 400-10 000 All values taken from [6, 12]

1.2.2. Transportation, legal aspects and public acceptance of storing

Even though EOR and storage in gas fields and aquifers are used today, none of these are used in connection to a power production plant. Hence, to be able to inject CO2 from power production, a new infrastructure must be built which may include pipelines and/or transportation with boats.

The CO2 would then preferably be transported as a liquid which requires that it is compressed after being separated from a combustion process. Transportation of CO2 is already well known when it comes to tankers, pipelines and injection techniques. As an example, more than 2500 km of CO2 pipelines have been in operation in North America since the 1970´s. [6, 12, 18, 20, 25-28]

Compared to natural gas or hydrogen, transportation of CO2 is less hazardous in case of leakage.

Firstly CO2 is not explosive and secondly CO2 is non-toxic in small quantities, but could however cause immediate dangers to human life and health at concentrations of 7-10 vol%, or higher.[12,

28] CO2 is cheaper to transport than the cost of transmitting electricity which means that it is economically favorable to locate power stations close to electricity demand rather than close to the source of CO2 storage.[6]

Another very important issue of CO2 storage, especially storage connected to aquifers or the sea, is the legal aspect.[12, 16, 17] Unfortunately there is not yet one consistent legal framework regarding CO2 storage. Problems of CO2 storage include whether CO2 should be regarded as a waste or not. Typically, CO2 from fossil fuel combustion is seen as a waste but not CO2 extracted from natural gas, as for example the gas injected in the Sleipner field. However, recent amendments to two very crucial marine conventions, the London Protocol (global) and OSPAR

(western European), legally opens up for storage of CO2 in subsoil formations.[17] Additional problem arises if one wants to store other residuals from combustion as well. As an example it is today believed that SO2 practically could be injected together with CO2, which will reduce the cost and complexity of a combustion plant with inherent capture.[2, 16] However, gaining acceptance for storing of SO2 is probably more difficult than for storing of CO2. Another difficult task to solve concerns the legal responsibility. Since many aquifers are situated in more than one country, who is responsible for the surveillance and monitoring and who pays the price in case of failure and potential environmental hazard? Since a clear legislation designated to CO2 storage does not exist, this issue can delay introduction of CO2 mitigation techniques.

A crucial issue that should not be forgotten is the public acceptance. Although researchers and policy-makers in general have a positive view on CCS, the public is to a large degree unaware of

8 planned storing of CO2, and when told about the possibilities, express concern for the safety of the transport and storing facilities.[29-31]. Furthermore, as in the case with introducing renewable energy sources, CCS will not be competitive without economic incentives and/or through environmental legislations. There will be extra cost involved for the consumer independent of which carbon-free technology that is employed. Hence, solving the global warming problem will result in increased energy prices. Therefore, as an example, it is quite alarming that 43% of the Swedes that participated in a recent survey refused to pay anything extra for their electricity in order to solve global warming.[31] It then becomes obvious that not only potentially controversial issues, such as storing of CO2, but also the higher cost of climate mitigation may need public acceptance.

However, when storage of CO2 is believed to be feasible, the remaining task is to separate CO2 from the combustion plant to the lowest cost and energy penalty possible.

1.3. Sequestration technologies

Several different technologies have been suggested in order to obtain CO2 in a pure stream from combustion in a power production unit. The three most common technologies are postcombustion, O2/CO2 firing (oxyfuel) and precombustion (CO-shift). [1, 2, 10, 12, 18, 32] A fourth option is unmixed combustion.

It would be simple to collect CO2 from a combustion plant if it came in a separate stream, however, depending on fuel and process, a typical concentration of CO2 in the flue gas of an existing power plant is 4-14% (lower values with natural gas and higher with coal) whereas the rest is mainly nitrogen from the combustion air.[1, 6, 8, 18] Therefore, the major energy consuming step inherent to all technologies is that of obtaining CO2 in a pure form. It is necessary to separate nitrogen from CO2 before storage for two reasons. Firstly, there is limited storage capacity, as mentioned in section 1.2. Secondly, the energy consumption for compression of the gases for transportation would otherwise increase dramatically due to the larger amounts of gas which needs to be treated.

A schematic picture of postcombustion can be seen in Figure 1-4. In postcombustion, CO2 is captured from the flue gases; in this way the combustion process in itself is not affected. This can for instance be done with amine absorption or membrane separation. A positive aspect of this

9 sequestration technique is that the addition of the equipment for CO2 removal may be added to a power plant. A negative aspect is the cost and high energy penalty.[12]

Figure 1-4 Schematic view of postcombustion

Another type of sequestration technology is O2/CO2 firing (also called oxyfuel), which can be seen in Figure 1-5. Nitrogen from the air is here separated before combustion in an energy consuming air separation unit. In this way the combustion products will mostly be CO2 and water, where the latter can easily be condensed. This also means that smaller amounts of flue gas will be present compared to posttreatment, as well as a much higher partial pressure of CO2 in the flue gas. Higher CO2 contents will however mean that the boiler is more sensitive to corrosion than in an ordinary air-fired boiler.[33] Compared to the post- and precombustion technologies which are expected to capture 85-90% CO2, the capture efficiency with oxyfuel is expected to reach almost 100%.[33] Another advantage with this technology is that NOx formation is suppressed.[10]

Figure 1-5 Schematic view of O2/CO2 firing (oxyfuel)

The third capture method is often referred to as precombustion, as seen in Figure 1-6. In precombustion the CO2 is separated before the combustion. This is achieved by having a first unit where air or O2 and/or H2O are added to the fuel which is converted to a mixture of CO2, CO,

H2O and H2. This mixture then reacts in a shift reactor to CO2 and H2 which is followed by a separation unit where CO2 is separated from H2. Hence, a hydrogen-rich fuel will be used for the final combustion and the combustion product will be mostly steam. The partial pressure of CO2 is much higher in this method than in posttreatment. Several methods could be possible for the separation of CO2 from H2, such as pressure swing adsorption, cryogenic, scrubbing or membranes, but again this step is energy consuming. Furthermore this technology is very complex and hard to fit into an existing system. Nevertheless, the technique itself is not new as it has been used in, for example, ammonia production.[10] Note that if the combustion step is excluded, this is a process for manufacturing of hydrogen from a (fossil) fuel.[1]

Figure 1-6 Schematic view of precombustion

A drawback with these three methods is the high energy penalty needed to achieve CO2 in a pure stream. An absolute decrease in efficiency of 7-15 %-units[6, 32, 34] and an increase in specific cost ($/kW) of 20-85%[2, 10, 32] have been mentioned when using natural gas or coal as fuel. The decrease in efficiency corresponds to an increase in fuel consumption; e.g. for plants capturing approximately 90% of CO2, an increase in fuel consumption/kWh of 10-40% could be expected depending on fuel and process.[12, 20] Hence, there is no doubt that electricity produced from a CCS unit will be more expensive than today, both because the separation and storing is an additional cost but also because more fuel will be needed per produced kWh electricity.

Regarding the additional costs for separation, transportation & storage of CO2 from any of the three methods described above, approximately 75-80% are associated with removal and compression of CO2 to a liquid (~100 bar), whereas the rest covers transportation and storage.[1, 18, 25] However, this is dependant on the transportation distance as some authors claim that the

11 cost of transportation of CO2 is 0.5-3 US$/tonCO2*100km and that the cost of storage in aquifers is 1-3 US$/tonCO2.[6, 25] This should be compared with an estimated cost of 30-50

US$/tonCO2 for separation using any of the separation technologies mentioned.[6, 25] Hence, it is clear that the cost of separation of CO2 from a power process is the single most costly part of the CCS chain. In general the costs and energy penalties are associated with the need to separate gases. It would be favorable if the nitrogen from the air is kept from being mixed with the fuel without the need for an excessive energy input. This could be achieved by a fourth method of

CO2 capture: unmixed combustion. This could be accomplished in chemical-looping combustion.

1.4. Chemical-Looping Combustion (CLC)

Figure 1-7 Schematic view of Chemical-Looping Combustion (CLC)

The concept of chemical-looping combustion (CLC) is based on the use of an oxygen carrier material. The carrier’s purpose is to transfer oxygen from the combustion air to the fuel. In this way any costly separation of nitrogen is avoided, since the uptake of oxygen from the air is achieved by a simple solid-gas phase reaction. A schematic picture of CLC can be seen in Figure 1-7. The combustor consists of two interconnected fluidized bed reactors; an air- reactor and a fuel-reactor. In-between these, the oxygen carrier particles are circulating. The oxygen carriers are oxidized in the air-reactor by taking up oxygen from the air. The particles are then transferred to

12 the fuel reactor where they are reduced due to combustion of the fuel with the oxygen from the oxygen carrier. Hence, in both reactors the reactions proceed through solid-gas phase reactions.

The reactions taking part in the different reactors are;

Fuel reactor: (2n+m)MyOx + CnH2m → (2n+m)MyOx-1 + mH2O + nCO2 (1)

Air reactor: (2n+m)MyOx-1 + (n+½m)O2 → (2n+m)MyOx (2) ______

Net reaction: CnH2m + (n+½m)O2 → mH2O + nCO2 (3)

As can be seen, the result of the two separate reactions is the same as for normal combustion. Reaction (1) is either endothermic or exothermic, depending on type of oxygen carrier and fuel, whereas reaction (2) is always exothermic. The advantage compared to normal combustion is that

CO2 is only diluted with steam in this process. Steam can be condensed and in this way CO2 is achieved in a separate stream. In principle, the fuel could be in gaseous, liquid or solid form, although the most likely initial application will be for gaseous fuels, such as syngas from gasification of coal, refinery gas or natural gas. Expected temperature range could be between 800-1200 °C and the combustor could be either pressurized or atmospheric.

An additional advantage with chemical-looping combustion is that NOx should not be generated in combustion. Thermal NOx in the air reactor is avoided due to the relatively low reaction temperature used for this application. The lack of thermal NOx in the exit gases has also been verified experimentally.[35, 36] For fuel NOx no verification yet exists, but conditions in the fuel reactor would not be excepted to be favorable for NOx formation.

A proposed design of a chemical-looping combustor was given by Lyngfelt et al.[37] This can be seen in Figure 1-8 and is based on interconnected fluidized beds. The main advantage of this type of system is that there is good contact between the oxygen carriers and the fuel. The air-reactor is a riser (#1 in figure) where the air flow is sufficiently high to entrain the particles. The particles are lead to a cyclone (#2) from where they are separated from the flue gas. From the cyclone the particles fall down into the fuel-reactor (#3), which is a bubbling fluidized bed. Via an overflow chute the particles are then transported back to the air-reactor, whereas the combustion gases

(CO2 and H2O) are separated in a condenser and finally the CO2 will be compressed to a liquid

13 for further transportation. There are particle locks installed between the reactors to prevent any mixing of the gases, i.e. keep the two gas streams of the fuel and air reactor separate. If there is remaining non-condensable combustible gas from the gas stream leaving the fuel reactor, one option would be to recover this gas and recycle it to the fuel reactor. Another option is to add some oxygen downstream of the fuel reactor. However, both options should try to be avoided, since they result in a lower overall efficiency.

The interconnected fluidized beds adapts technology proven to work in other applications, e.g. circulating fluidized bed (CFB) boilers used for solid fuels.[37, 38] Instead, what is new and crucial, is the use of the oxygen carriers. These particles should react fast with fuel and oxygen respectively, be able to convert the fuel to CO2 and H2O to a high degree as well as being able to withstand physical damage over a large number of cycles. In the end, the particle production cost in combination with the lifetime of the particles could be decisive when the costs of chemical- looping combustor are compared to those of other separation techniques.

Figure 1-8 Proposed design of a chemical-looping combustor.

Although most work on chemical-looping combustion focus on the design with interconnected fluidized beds, other reactor systems have been proposed, e.g. using a monolithic rotating reactor [39] or a packed bed reactor system.[40]

1.4.1. Integration with power process and thermal efficiencies

The principle of chemical-looping combustion was first proposed in a patent 1954 as a way to produce pure CO2 from fossil fuels.[41] In 1983, Richter and Knocke presented CLC as a way to increase the thermal efficiency of a power plant.[42] Since the mid 90s, when CLC was proposed as a way of separating CO2 in a combustion plant by Ishida et al [43], research on chemical- looping combustion has increased rapidly. The work has mainly been focused on oxygen carrier development and testing (see section 1.4.6), but considerable work has also been performed around reactor design and thermal performance.

It is important that the chemical-looping system in Figure 1-8 can be integrated with a power process and achieve high efficiencies. There has been a number of process simulations performed in the literature using both natural gas and syngas and different types of oxygen carriers. A review of the literature around these process simulations can be found in doctoral theses of Anheden [44], Wolf [45], Brandvoll [46] and Naqvi [47]. As mentioned above chemical-looping combustion was once proposed as a combustion technique for increasing the thermal efficiency of combustion. It has been claimed that the exergy destruction in such a process is less in comparison to normal combustion.[42, 48, 49] By performing the reactions in two steps, the inherent disorder of normal combustion is avoided and hence if the added exergy can be utilized in a good way, higher thermal efficiencies should be obtained. In the first set of systems analyzed, the capture of CO2 was not incorporated, and electrical efficiencies of between 50 – 67% based on the lower heating value of the fuel were reported, see [44]. Later, Anheden et al. found that it was theoretically possible to increase the efficiency using simple gas turbine systems incorporated with CLC, but that CLC together with a gas and steam turbine cycle did not have any efficiency improvement in comparison to normal combustion.[50-52] However, if CO2 capture was added, the CLC combined cycle systems showed higher efficiencies compared to “conventional” systems with CO2 capture. Later process studies have focused on CLC with CO2 capture. Wolf et al. performed process studies on natural gas combined cycle (NGCC) systems and found that the thermal efficiency could be increased by 5 percentage points by using CLC in comparison to conventional CO2 capture technology.[53] The group of Bolland et al. has also performed several studies of natural gas fired cycles with different configurations, and in general the thermal efficiencies are high.[54-56] In conclusion, the process studies have shown that it is theoretically possible to achieve high thermal efficiencies using CLC integrated with CO2 capture, almost always superior to alternative methods. This together with the added advantage that no new

15 separation equipment is needed and hence, considerably smaller capital costs, makes CLC a highly interesting technology for further study.

In the investigations presented above it is usually assumed that the reactions in the reactors are in equilibrium, which implicitly assumes that the oxygen carriers react at a rapid rate with the fuel and oxygen. Further, no aspects concerning the behaviour of the oxygen carrier particles in the reactors are taken into account, i.e. deactivation, agglomeration and attrition. And as the temperatures employed in the process studies are usually in the excess of 1000°C in the air reactor, these aspects may be of critical importance. Finally, little or no information concerning reactor design is given. Thus, to reach the high efficiencies calculated above, it is crucial that reactor configurations and oxygen carrier particles are developed which can enable integration into a highly efficient power cycle.

1.4.2. Chemical-Looping Reforming (CLR) The chemical-looping technique can also be adapted for the production of hydrogen with inherent CO2 capture. Two processes have been compared by Rydén and Lyngfelt: i) Autothermal chemical-looping reforming, CLR(a) and ii) steam reforming using chemical-looping combustion, CLR (s).[57, 58]

CLR(a) is similar to CLC, but instead of completely burning the fuel, it is partially oxidized using a solid oxygen carrier and some steam to produce an undiluted stream of H2, CO, H2O and CO2, see Figure 1-9.[58, 59] The actual composition of this mixture depends upon the air ratio, i.e. the ratio of oxygen supplied to the fuel by the oxygen carriers in the fuel reactor to that needed for complete oxidation. This gas mixture could then be converted to a mixture of pure H2 and CO2 in a low temperature shift-reactor. Depending upon the purity of H2 required and the pressure, the CO2 can be removed by either absorption or adsorption processes.

Figure 1-9 Schematic view of Autothermal Chemical-Looping Reforming (CLR(a))

The second type of hydrogen production technique is called CLR(s) where the “s” denotes steam reforming. Here, natural gas is converted to syngas by conventional steam reforming, i.e. the natural gas reacts with steam at high pressures inside tubes containing suitable catalysts. However, the steam reforming tubes are placed inside the fuel-reactor of a CLC unit. Hence, in contrast to the normal steam reforming process, the reformer tubes are not heated by direct firing but rather by the oxygen carrier particles in the normal CLC process. The syngas passes through a shift-reactor and a condenser before high purity H2 is obtained through pressure swing adsorption (PSA). The offgas from the PSA unit, consisting of a mixture of CH4, CO2, CO and

H2, is then the feed gas to the fuel reactor. The proposed design of CLR(s) can be seen in Figure 1-10.[57]

Figure 1-10 Schematic view of Steam reforming using Chemical-looping combustion (CLR(s))

The two CLR concepts above have been compared in a process study, in which CO2 capture has been considered. It was found that both alternatives have potential to achieve reforming efficiencies in the order of 80%, including CO2 capture and compression.[59]

Other authors have explored the possibility of using oxygen storage materials for the production of syngas in processes similar to CLR(a), e.g. [60-62].

1.4.3. Chemical-Looping Combustion of solid fuels

Although considerable work has been carried out in the last decade on CLC, most of the investigations have focused on natural gas as fuel. As coal is a much more abundant and less costly fossil fuel, it would be highly advantageous if CLC could be adapted for coal combustion. Two possibilities for achieving this are:

• introduction of the coal directly to the fuel reactor where the gasification of the coal and subsequent reactions with the metal oxide particles will occur simultaneously in the same reactor • using syngas from coal gasification in the fuel reactor. Here, an energy intensive air separation unit (ASU) would be needed for the gasification step, but only a limited part of the total oxygen required for complete combustion would have to be produced in the ASU.

Many experiments assuming the second route with reaction between syngas and oxygen carriers have been performed, including Paper IX in this thesis. From these studies it appears that oxygen carriers are generally more reactive towards both hydrogen and carbon monoxide or the mixture of these (syngas) in comparison to natural gas or methane.[63-65]

The first route; direct combustion between different types of coal and oxygen carriers has only been tested by a few authors.[66-71] Of these authors, only Leion et al. used an synthetically produced oxygen carrier (iron oxide) that was developed using gaseous fuels (# 25 in Table 1-6). [71] Steam can be introduced both in order to fluidize the bed and gasify the coal. Since gasification is assumed to be the rate determining step, there is less focus on finding oxygen carriers with high reactivity. There are also good reasons to assume a shorter lifetime of the oxygen carriers when used with solid fuels, which also makes cheap material more interesting. Further on, more advanced design criteria of the reactor system are needed compared to when using a gaseous fuel, due to the slowly diminishing size of the solid fuel particles particles.

Another option of chemical-looping combustion with solid fuels is to use biomass or municipal waste as tested by Cao et al.[68] The advantage of using biomass in “carbon capture and storage” for power production is that you are actually producing electricity and lowering the CO2 in the atmosphere at the same time. This is because burning of biomass ideally does not contribute to a net increase of CO2 in the atmosphere providing that you cultivate new crops in place of the old ones.[1, 12, 72]

1.4.4. Experiments of chemical-looping combustion and reforming in prototype units

Several CLC prototypes have been presented in the literature, see Table 1-3. Lyngfelt et al. presented results from a 10 kW prototype unit in 2004.[73, 74] Here, an oxygen-carrier based on nickel oxide (#74 in Table 1-9) was operated for 100 h with natural gas as fuel. A fuel conversion efficiency of 99.5% was achieved, and no carbon dioxide escaped to the air reactor, hence, all carbon dioxide was captured in the process. Only small losses of fines were observed.[75] It was estimated that the cost of the oxygen carrier was less than 1€/ton CO2 captured.[74] For a shorter period an iron oxide (# 25 in Table 1-6 ) was also tested in the same unit.[74] Recently, Linderholm et al used the same chemical-looping combustor for a 160 h test with natural gas as fuel and a nickel-based oxygen carrier. A fuel conversion to CO2 of more than 99% was accomplished and the amounts of fines leaving the reactor system were low at the end of the tests.[76] Ryu et al. have presented results from a 50 kW combustor operating with methane as fuel, and two types of oxygen-carriers.[77] A nickel oxide oxygen-carrier was tested during 3.5 h and a cobalt oxide was tested during 25 h. For the nickel oxide oxygen-carrier, the concentration based on dry flue gases of CO2 leaving the fuel reactor was 98% and for cobalt oxide 97%. The 10 kW and 50 kW reactors have a similar design, but differ at the return from the fuel reactor. In the 10 kW unit at Chalmers the particles leave the fuel reactor through an overflow, i.e. the bed height in the fuel reactor is always constant, while in the 50 kW unit in South Korea the particles leave the fuel reactor from the bottom of the bed, and the particle flow i.e. the bed height of the fuel reactor, is controlled by a valve. Adanez and co-workers have also presented results from a 10 kW CLC unit working with a copper-oxide based oxygen carrier. Two different particle sizes were tested for 60 hours each. Complete methane conversion was achieved and no deactivation of the particles was noticed.[78, 79] Song and Kim presented results with the mixed oxide carrier of NiO-Fe2O3/Bentonite in a circulating fluidized bed reactor using methane at a thermal power

19 of about 1 kW. Almost full conversion to CO2 and H2O was achieved; however no information was given concerning the length of the experiments.[36] The first tests conducted in a continuous chemical-looping combustor using solid fuels have recently been performed by Berguerand & Lyngfelt.[80, 81] In these experiments, an iron-titania ore, ilmenite, has been used as an oxygen carrier for the combustion of pet coke and South African coal.

Moreover, oxygen carriers based on Ni (#74 & 86 in Table 1-9), Mn (#57 in Table 1-7) and Fe (#12 in Table 1-6) have been used in a 300 W CLC reactor with both syngas and natural gas.[82- 85] The same reactor was also used with a nickel oxide (#86 in Table 1-9) in testing of CLR(a).[86] This reactor was designed specifically for testing smaller amounts of oxygen carrier material in a continuous fashion and was based on a cold-flow model tested by Kronberger et al.[87]

Table 1-3 Testing in chemical-looping combustors Combustion h Unit Oxygen carrier Fuelb Ref. (hot timea) a Chalmers 10 kW NiO/NiAl2O4 105 (300 ) n.g. [73, 74]

Chalmers 10 kW Fe2O3-based 17 n.g. [74]

S Korea 50 kW Co3O4/CoAl2O4 25 n.g. [77] S Korea 50 kW NiO/bentonite 3d n.g. [77] a Chalmers 300 W NiO/NiAl2O4 8 (18 ) n.g. [82] a Chalmers 300 W NiO/MgAl2O4 30 (150 ) n.g./s.g. [82, 83] a Chalmers 300 W Mn3O4/ ZrO2, Mg-stab. 70 (130 ) n.g./s.g. [84] c c Chalmers 300 W NiO/MgAl2O4 41 (CLR) n.g.(CLR) [86] a CSIC, 10 kW CuO/Al2O3 2x60 (2x100 ) n.g. [78, 79] a Chalmers 300 W Fe2O3/Al2O3 40 (60 ) n.g./s.g. [85]

S Korea, 1 kW NiO-Fe2O3/bentonite ? CH4 [36]

Chalmers 10 kW NiO/NiAl2O4 160 n.g. [76] Chalmers 10 kW s.f.e Ilmenite 22 (140a) RSA Coal [80] Chalmers 10 kW s.f.e Ilmenite 11 Pet coke [81] a total time fluidized at high temperature, bn.g. = natural gas, s.g. = syngas, c Autothermal chemical-looping reforming, dparticles fragmentated, esf = unit designed for solid fuels

1.4.5. Oxygen Carriers Most of the work on chemical-looping technologies has been focused on the development and testing of oxygen carriers. Initial ideas for suitable oxygen carrier material for CLC and CLR(a,s) were mainly taken from the research area of heterogeneous catalysis used for reforming of hydrocarbon fuel. However, it is important to point out that knowledge obtained from research on catalysts for reforming is insufficient. The reason for this is that both CLC and CLR(a,s) are based on primary non-catalytic reactions and that the oxygen carriers act as a source of undiluted

20 oxygen (i.e. without nitrogen). Even though the primary focus of CLC and CLR(a,s) differs, the oxygen carrier particles carry heat and oxygen from the air reactor to the fuel reactor in all three technologies. Because of the need to transfer large amounts of oxygen between the air and fuel reactor, the oxygen carriers for chemical-looping technologies have high ratios of active material to inert material (typically 20-80%), as compared to heterogeneous catalysts where the fraction of active material typically is less than 10%.

Almost all research on oxygen carriers has been directed towards finding suitable materials for CLC. For CLR (a) only a limited amount of papers exist.[86, 88, 89]. For CLR(s) the fuel feed mixture consists of reactive CH4, CO and H2 and unreactive CO2. Earlier studies of oxygen carriers clearly indicate that methane is much more difficult to convert than CO and H2.[63-65] Therefore, the development of oxygen carriers for burning methane-rich fuels in CLC may be highly relevant also for CLR(s).

For the kind of fluidized bed systems outlined above, the criteria for a good oxygen carrier are the following: • High reactivity with fuel and oxygen • Low fragmentation and abrasion • Low tendency for agglomeration • Low production cost and preferably being environmentally sound. For CLC and CLR(s) you have the additional requirement:

• Able to convert the fuel to CO2 and H2O to the highest degree possible (ideal 100%)

Three parameters related to the oxygen carrier are important to establish in order to design a CLC reactor system. These parameters and their relation to design data are shown in Figure 1-11.

The bed mass in a real system is inversely proportional to the reaction rate of the oxygen carriers. A high reaction rate means a smaller bed mass needed and thus smaller reactor sizes and less material production costs. The solids recirculation in a real system is dependent on the difference in conversion, ΔX, between particles in the fuel-and air reactor. If a particle can maintain high reaction rate through a wide conversion range – a smaller recirculation rate may be required which may be an advantage. However, with respect to oxygen carrier recirculation, it is often necessary to also consider the thermal aspects. As seen in Table 1-4, three out of four oxygen carrier types investigated have endothermic reaction with methane. This means that if hot

21 particles from the air-reactor do not enter the fuel-reactor at a sufficient rate, the temperature drop may be so high that the rate of reaction decreases to an unacceptable level.[90] The correlation between temperature drop and recirculation rate is demonstrated for some iron based oxygen carriers in Paper III. The third crucial parameter is the conversion, or gas yield, of the fuel. Unconverted gas from the fuel reactor must either be re-circulated back or burned off by adding oxygen downstream of the fuel reactor. However, these options are associated with extra costs and added complexity of the system and should be avoided if possible.

Figure 1-11 The relations between carrier reactivity and design input data. Taken from [37]

With respect to the ability of the oxygen carrier to convert a fuel gas fully to CO2 and H2O for CLC, Mattisson and Lyngfelt investigated the thermodynamics of a few possible oxygen carriers and concluded that the metal oxide/metal (or metal oxide of lower oxidation state) systems of

NiO/Ni, Mn3O4/MnO, Fe2O3/Fe3O4, Cu2O/Cu, CoO/Co were feasible to use as oxygen carriers for the temperatures of interest.[91] A comprehensive study was made by Jerndal et al., where 27 different possible systems for CLC were investigated with respect to thermodynamics, melting points, oxygen ratio, carbon deposition and fate of possible sulfur species in the fuel.[90] Again, the same metal oxides were mentioned as suitable candidates. The often studied NiO/Ni system has a slight disadvantage in that the system can not convert a hydrocarbon fuel completely to CO2 and H2O. However, the conversion is still very high, 98.8 % at 1000 °C, and higher at lower temperatures. For CoO/Co the same problem exists, however with considerably less favorable thermodynamics, the conversion decreases with temperature and is only 93.0 % at 1000

°C. In practice it means that the CO2 will contain combustible gases, i.e. CO and H2, if these systems are used.

Figure 1-12 Amount of active material in different unsupported oxygen carrier material.

The oxygen transfer capacity, i.e. the ratio of mass of active oxygen in the carrier to the mass of the fully oxidized oxygen carrier, for some of the different systems can be seen in Figure 1-12.

Included in this figure is the amount of oxygen for Fe2O3/Fe, which is significantly higher than 0 Fe2O3/Fe3O4. The reason why a transition to pure Fe or FeO was not of interest in studies regarding CLC, is the thermodynamical limitations for converting the fuel completely to CO2 and

H2O, which limits its use for CLC and CLR(s). [90] The reason why Mattisson and Lyngfelt and

Jerndal et al described Cu2O/Cu as the proposed system for copper is that CuO can decompose to Cu2O, depending on the reactor temperature and partial pressure of oxygen. As an example, if the partial pressure of oxygen in the air reactor is 4%, which is a valid assumption in the air reactor in CLC, a temperature of 944°C or higher means that CuO decomposes. Because of the low melting temperature of Cu, in practice a lower temperature may need to be used in a CLC system with this oxygen carrier and thus the active system will be CuO/Cu, which has a higher amount of available oxygen.

Different key properties of the four metal oxide systems investigated in this thesis are presented in Table 1-4. The values of the corresponding gas yields expressed, γi, are based on thermodynamical limitations.

Table 1-4 Properties of the most suitable oxygen carriers

γCH4 γCO γH2 Melting ΔH/ ΔH/ ΔH/ MeO/ Ro 800/ 800/ 800/ temp. (°C) ΔHdir ΔHdir ΔHdir Me (eq 6) 1000°C 1000°C 1000°C † ‡ § Oxid./Red. comb CH4 comb CO comb H2 (eq 11) (eq 12) (eq 13)

Fe2O3/ 1.0000/ 1.0000/ 1.0000/ 1565/1597 0.033 1.19 0.85 0.96 Fe3O4 1.0000 1.0000 1.0000 Mn3O4/ 1.0000/ 1.0000/ 1.0000/ 1562/1842 0.070 1.12 0.80 0.90 MnO 0.9999 0.9999 0.9999 CuO/ 1.0000/ 1.0000/ 1.0000/ 1446/1085 0.201 0.74 0.53 0.59 Cu 1.0000 1.0000 1.0000 NiO/ 0.9949/ 0.9949/ 0.9946/ 1955/1455 0.214 1.17 0.83 0.94 Ni 0.9883 0.9883 0.9931 All data taken from Jerndal et al.[90]

Reactivity experiments simulating chemical-looping combustion performed on natural ores or pure metal oxides, i.e. without addition of inert, have shown fast degeneration or low reactivity of these material.[63, 92-95] In order to achieve highly reactive and stable oxygen-carriers, these could be prepared synthetically and mixed with an inert material. The inert material is believed to increase the porosity of the particles, help to maintain the structure and possibly also increase the ionic conductivity of the particles. Even though the ratio of free oxygen in a particle decreases with the addition of inert material, the reactivity with the fuel and oxygen can still be higher due to the increased porosity [92], which also was shown for the iron oxides investigated in Paper VI.

1.4.6. Oxygen Carriers in literature

During the last decade a lot of research on oxygen carriers for chemical-looping combustion has been performed. The major contributors have been CSIC in Zaragoza, Spain, Chalmers University of Technology in Göteborg, Sweden, Tokyo Institute of Technology in Japan and

† o~íáç=ÄÉíïÉÉå=êÉ~Åíáçå=ÉåíÜ~äéó=çÑ=çñóÖÉå=~åÇ=ãÉí~ä=çñáÇÉ=~åÇ=êÉ~Åíáçå=ÉåíÜ~äéó=çÑ=ÇáêÉÅí=êÉ~Åíáçå=ÄÉíïÉÉå=çñóÖÉå= ~åÇ=ãÉíÜ~åÉ=E~í=NMMMø`FK=eÉåÅÉI=~=åìãÄÉê=ÄÉäçï=N=áåÇáÅ~íÉë=ÉñçíÜÉêãáÅ=êÉ~Åíáçå=áå=ÑìÉä=êÉ~Åíçê=~åÇ=~=åìãÄÉê= ÜáÖÜÉê=íÜ~å=N=áåÇáÅ~íÉë=ÉåÇçíÜÉêãáÅK== ˛=^ë=~ÄçîÉI=Äìí=ïáíÜ=`l= ¬ =^ë=~ÄçîÉI=Äìí=ïáíÜ=eO 24

Korea Institute of Energy Research. A review of the papers describing the particles investigated so far can be found in Table 1-5. The papers are ordered chronologically except papers published the same years which are in alphabetical ordered based on the first author. Excluded from Table 1-5 are papers that are smaller versions of papers already listed or summaries of results already included in the listed papers.[73, 91, 96-113]

Table 1-5 Literature on experimental data on oxygen carriers for chemical-looping technologies Ref Oxygen carrier # O.C. Reference Reduction agent Tred (°C) Dp (mm) Apparatus Notes # (MexOy/support) (new) Nakano et Fe2O3, Fe2O3-Ni, a [114] 3(3) H2, H2O/H2 700-900 0.007 TGA a al. 1986 Fe2O3/Al2O3 Ishida and NiO, NiO/YSZ, [92] 3(3) H2, H2O/H2 550 - 950 1.3 - 2.8 TGA b, c Jin 1994 Fe2O3/YSZ Ishida et al. 600, 800, 1.8, (1.0 - [115] NiO/YSZ 5(4) H c TGA c, u 1996 2 1000 3.2) Ishida and [116] NiO, NiO/YSZ 2(0) H 600 2 TGA d Jin 1996 2 Hatanaka et [117] NiO 1(1) CH 400 - 700 0.074 FxB al. 1997 4

Ishida and NiO/YSZ, NiO/Al2O3, 600, 700, [118] 3(1) H2, CH4, H2O/CH4 2 TGA e Jin 1997 Fe2O3/YSZ, 750 NiO/YSZ, NiO/Al2O3, H2/N2, CO/N2, Ishida et al. NiO/TiO2, [119] 10(6) CO/N2/CO2, 550- 900 1.6 TGA e 1998 Fe2O3/YSZ, Fe2O3/ CO/N2/H2O Al2O3, Fe2O3/TiO2 NiO/YSZ, Jin et al. Fe O /YSZ, [120] 2 3 4(2) H , CH 600 1.8 TGA e 1998 CoO/YSZ, CoO- 2 4 NiO/YSZ Ishida et al. 600, 900, [121] NiO/NiAl O 1(1) H 0.097 CFzB 1999 2 4 2 1100

NiO/Al2O3, NiO/TiO2, NiO/MgO, NiO/NiAl2O4 , NiO/YSZ Jin et al. [122] CoO/Al O , 11(5) H , H O/CH 600, 700 1.8 TGA e, f 1999 2 3 2 2 4 CoO/TiO2, CoO/MgO Fe2O3/ Al2O3, Fe2O3/ TiO2, Fe2O3/MgO Stobbe et al. [61] Manganese Oxides 4(4) CH4/Ar, H2/Ar 20-827 0.15-0.5 - m, t 1999

CuO-based, Fe2O3- based Copeland et [123] on alumina, 30(30) CO /H /CH 800 Fine powder TGA al. 2000 2 2 4 aluminates and silicates j Mattisson et Fe2O3 , Fe2O3/ Al2O3, [95] 6(6) CH4 950 0.12-0.50 FxB al. 2000 Fe3O4 Copeland et Fe O -based, NiO- i [124] 2 3 ? H /CH , Syngas 720-1050 - TGA, FzB al. 2001 based 2 4 Jin and NiO, NiO/YSZ, 1.8, 2.1, [125] 3(0) H2, H2/Ar 600 g TGA, FxB m Ishida 2001 NiO/NiAl2O4 4.0×1.5 Mattisson et j [93] Fe O 1(0) CH 950 0.18-0.25 FxB al. 2001 2 3 4 Ryu et al. NiO/bentonitek, [126] 4(4) CH /N 650 - 900 0.080 TGA u 2001 Ni/bentonitel 4 2

Cho et al. Fe2O3/Al2O3, 0.125-0.18, [127] 4(4) CH4 950 FzB 2002 Fe2O3/MgO 0.18-0.25 Copeland et Fe O -based, NiO- i [128] 2 3 ? Syngas 780 - FzB al. 2002 based Ishida et al. h [129] NiO/NiAl O 1(0) H , H /Ar 600 -1200 0.097 TGA, CFzB h 2002 2 4 2 2 Jin and NiO/YSZ, 600, 700, g [130] 3(0) H2O/CH4 f 4.0=1.5 TGA, FxB e, f Ishida 2002 NiO/NiAl2O4, CoO- 800

NiO/YSZ Ryu et al. [131] NiO/bentonite 1(1) CH /N 650 - 1000 0.091 TGA e 2002 4 2

NiO/TiO2, Johansson g [132] Fe O /TiO , CuO/ 18(18) CH , H O/CH 700- 900 1.5-2=2.5-3 TGA M. 2002 2 3 2 4 2 4 TiO2, MnO2/ TiO2 Adánez et [133] CuO/SiO 1(1) CH 600-850 1 TGA al. 2003 2 4 0.3-0.5 Brandvoll et 0.6-1.0 al. [134] NiO/NiAl O 3(3) H 600-850 FxB/FzB u 2 4 2 1.2-1.7 2003 2.0-3,5

Jeong et NiAl2O4, CoAl2O4, s al. [135] CoOx/CoAl2O4, 8(8) H2/Ar, CH4/Ar/He 150-1000 - TGA s,m 2003 NiO/NiAl2O4 s NiO/YSZ, CoO/YSZ, Lee et al. [136] Fe O /YSZ, NiO- 4(4) - - - TGA s 2003 2 3 Fe2O3/YSZ NiO/Al2O3, Mattisson et CuO/Al2O3, 750, 850, [137] 4(4) H2O/CO2/ CH4 /N2 0.1-0.5 TGA al. 2003 CoO/Al2O3, 950 Mn3O4/Al2O3 Ryu et al. 0.091, [138] NiO/bentonite 1(0) CH /N , H 500 - 1000 TGA, FxB e 2003 4 2 2 0.128, 0.4 NiO/Bentonite, NiO/YSZ, Ryu et al. s [139] (NiO+Fe O )/YSZ, 5(4) H /N , CH /N 50 -1000 - TGA m 2003 2 3 2 2 4 2 NiO/NiAl2O4, CoxOy/COAl2O4 Ryu et al. [140] NiO/bentonite 1(0) CH /N 650 -1000 0.091 TGA e 2003 4 2 Song et al. [141] NiO/hexaaluminate 5(5) H /Ar 25 – 1000 - TGA m 2003 2 Villa et al. NiO /NiAl O , Ni H , CH /He, CH , 800, 25 – e, m, [142] x 2 4 1- 6(6) 2 4 4 - TGA 2003 yMgyAl2O4 CH4/H2O 1000 v CuO, Fe2O3, MnO2, Adánez et NiO with Al2O3, 240 g [143] CH4/H2O 800, 950 2=4 TGA al. 2004 sepiolite, SiO2, TiO2, (225) ZrO2 CuO, Fe O , MnO , Adánez et 2 3 2 [144] NiO with Al O , SiO , 26(2) CH /N 800, 950 0.1-0.3 TGA, FzB al. 2004 2 3 2 4 2 TiO2, ZrO2 CuO, MgO, TiO2, Chahma et Al2O3, CuO/MgO, 550, 650, [145] 7(7) CH4 - TGA al. 2004 CuO/TiO2, 750 CuO/Al2O3

Fe2O3/Al2O3, Fe O /Kaolin, Cho et al. 2 3 [146] NiO/NiAl O , 7(7) CH /H O 850, 950 0.125-0.18 FzB k 2004 2 4 4 2 CuO/CuAl2O4, Mn3O4 with MnAl2O4 CuO with Al2O3, de Diego et CH4, H2, or CO/H2 [63] sepiolite, SiO2, TiO2, 19(4) 800 0.2-0.4 TGA al. 2004 in H2O ZrO2

García- CH4/CO2/H2O, Labiano et [64] CuO/Al2O3 1(1) H2/CO2/H2O 500-800 0.1-0.3 TGA u al. 2004 CO/CO2/H2O CO/H2/H2O/Ar/CO2 Jin and NiO/NiAl O , CoO- g [147] 2 4 2(0) , CO/H /H O/Ar, 600, 700 4.0×1.5 FxB f Ishida 2004 NiO/YSZ 2 2 CH4/H2O

Johansson 0.09-0.125 M et al. [148] Fe O /MgAl O 15(11) CH /H O 650-950 FzB c 2 3 2 4 4 2 0.125-0.18 2004 0.18-0.25

Lee et al. NiO with AlPO4, [149] 7(6) H2 600 - TGA 2004 ZrO2, YSZ, NiAl2O4 Fe O with Al O Mattisson et 2 3 2 3 [150] (some with kaolin), 27(26) CH /H O 950 0.125-0.18 FzB al. 2004 4 2 ZrO2, TiO2, MgAl2O4 Mattisson et [151] CuO/SiO 1(1) CH /H O 800 0.18-0.25 FzB t al. 2004 2 4 2 Ryu et al., NiO/bentonitek, [152] 5(0) CH /N 25-1000 0.081, 0.091 TGA, FxB s 2004 s Ni/bentonitel 4 2

Ryu et al. NiO/bentonite, [77] 2(1) CH4 750, 869 0.106-0.212 CFzB h,o 2004 CoxOy/CoAl2O4 0.02-0.2 Brandvoll NiO/NiAl2O4, 600, 700, [46] n 2(1) H , CH , CH /H O 0.09-0.2 FxB/FzB 2005 Perovskite 2 4 4 2 800 0.4-2.6 Cao et al, m [67] CuO 1(1) Coal 50-900 - TGA m,x 2005

Cho et al. Fe2O3/Al2O3, 750, 850, [153] 2(0) CH4, CH4/H2O 0.125-0.18 FzB e 2005 NiO/NiAl2O4 950 Corbella et 100-950m [154] CuO, CuO/TiO 6(5) H /Ar, CH 0.2-0.4 FxB m al. 2005 2 2 4 800, 900 Corbella et 100-1000m, [155] NiO, NiO/TiO 5(5) H /Ar, CH /Ar 0.2-0.5 FxB e, m al. 2005 2 2 4 900 de Diego et [156] 14(14) CH4/N2, H2 800, 950 0.1-0.32 TGA, FzB p al.,2005 CuO/Al2O3 De los Rios 0-700m, [157] Co TiO 2(2) H /Ar - TGA m, t et a, 2005 x y 2 700 Gupta et al, m [66] Fe O ,Fe-Ti-O 2(2) Coal, H /N 0-900 - TGA m,x 2005 2 3 2 2 Ishida et al, [158] Fe O /Al O 7(7) H 900 0.07 TGA v 2005 2 3 2 3 2

CoO/YSZ, Fe2O3/ Lee et al. YSZ, NiO, NiO with [94] 7(0) H2 600 2 TGA 2005 ZrO2, YSZ, AlPO4, NiAl2O4 Lyngfelt and NiO based, Fe O Thunman [74] 2 3 2(2) Natural gas 560-900 - CFzB h,r based 2005 Readman et n [159] Perovskite 1(1) H /He 800 - TGA al., 2005 2

NiO and Fe2O3 on Son and TiO , Al O and [160] 2 2 3 9(9) CH /H O/CO 650-950 0.106-0.15 TGA u Kim 2005 bentonite 4 2 2 NiO-Fe2O3/bentonite Zafar et al. NiO, CuO, Mn2O3, [88] 4(3) CH4/H2O 700-950 0.18-0.25 FzB t 2005 Fe2O3 with SiO2 Abad et al. Natural gas, [84] Mn O /Mg-ZrO 1(0) 800-1000 0.09-0.212 CFzB h, q 2006 3 4 2 Syngas

NiO/Al2O3, CH4/H2O/N2 Adánez et CuO/Al O , NiO- (TGA), CH or CO TGA, FxB, [161] 2 3 22(22) 4 950 0.1-0.3 al 2006 CuO/Al2O3 some or H2 (FxB) , FzB with K2O or La2O3 CH4/N2 (FzB) Adánez et 0.1-0.3, 0.2- [78] CuO/Al O 1(1) CH 700-800 CFzB r al 2006 2 3 4 0.5 PRB Coal, Wood, Cao et al, [68] CuO 1(0) Polyethene with 0-1000 0.050-0.150 TGA x 2006 N2 & CO2 Fe O /Al O , Cho et al. 2 3 2 3 [162] NiO/NiAl O , 6(4) CH 950 0.125-0.18 FzB p 2006 2 4 4 Mn3O4/Mg-ZrO2 Corbella et CH4, CH4/N2, 900, 0- [163] NiO/TiO2 2(2) m m 0.2-0.4 FxB m al, 2006 H2/Ar 950 m Corbella et CH4, CH4/Ar , 800, 0- [164] CuO/SiO2 3(3) m m 0.2-0.4 FxB m al, 2006 H2/Ar 950 Dennis et 0.300-0.425, [69] Fe O 1(1) Lignite Char+ H O 900 FzB x al. 2006 2 3 2 0.425-0.710 García- Fe O /Al O , H /N , CO/CO /N , 2 3 2 3 2 2 2 2 800, 450- Labiano et [165] NiO/NiAl O , 3(1) CO/H O/CO , 0.15-0.2 TGA f,u 2 4 2 2 950 al. 2006 CuO/Al2O3 H2/H2O/CO2 Johansson E. et al. [83] NiO/MgAl2O4 1(0) Natural gas 800 - 950 0.09-0.212 CFzB h,q 2006 Johansson NiO/MgAl O , NiO Natural gas, E. et al. [82] 2 4 2(0) 800 - 950 0.09-0.212 CFzB h,q based Syngas 2006

Johansson Fe2O3, Mn3O4 and M. et al. [166] NiO on different 50(19) CH4/H2O 950 0.125-0.18 FzB 2006 inerts

Johansson Fe2O3, Mn3O4, CuO M. et al. [167] and NiO on different 58(15) CH4/H2O 950 0.125-0.18 FzB 2006 inerts

Johansson Mn3O4 on ZrO2, Mg- [168] 15(14) CH4/H2O 950 0.125-0.18 FzB M. et al. ZrO2, Ca-ZrO2 and

2006 Ce- ZrO2 Johansson M. et al. [75] NiO/NiAl2O4 1(0) CH4/H2O 950 0.09-0.125 FzB 2006 Johansson NiO/MgAl O , M. et al. [169] 2 4 2(0) CH /H O 650-950 0.125-0.18 FzB Fe O /MgAl O 4 2 2006 2 3 2 4 NiO-bentonite, NiO- Liu et al. [170] barium- 4(4) CO/CO /H in He 700-900 - TGA 2006 2 2 hexaalumunite

Mattisson et NiO with NiAl2O4, [171] 19(19) CH4/H2O 950 0.125-0.18 FzB k, w al. 2006 MgAl2O4, TiO2, ZrO2 NiO/MgAl O , Mattisson et 2 4 [65] Mn O /Mg-ZrO , 3(1) Syngas, CH 650-950 0.18-0.25 FzB al. 2006 3 4 2 4 Fe2O3/Al2O3 Readman et [172] NiO/NiAl O 1(1) H /Ar, CH /He 800 0.09-0.21 TGA u al. 2006 2 4 2 4

CaO, CuO, Fe2O3, MgO, MnO2, NiO, TiO2, Al2O3, Fe2O3/ Al2O3, Fe2O3/ TiO , Fe O / MgO, Roux et al. 2 2 3 0.0019- [173] Fe O / CaO, NiO / 19(13) CH 550-950 TGA 2006 2 3 4 0.093 Al2O3, NiO / TiO2, NiO / MgO, NiO / CaO, CuO / Al2O3, CuO / TiO2, CuO / MgO Rydén et al Natural gas [86] NiO/MgAl O 1(0) 820-930 0.09-0.212 CFzB h,q,t 2006 2 4 (+steam) Scott et al Lignite + 0.300-0.425, [70] Fe2O3 1(0) 900 FzB x 2006 H2O/CO2/N2 0.425-0.710 NiO and Fe2O3 on Son and TiO , Al O and CH /H O/CO /N [36] 2 2 3 9(0) 4 2 2 2 650-950 0.106-0.15 TGA, CFzB d,u,h Kim 2006 bentonite (TGA) CH4 (CFzB) NiO-Fe2O3/bentonite Song et al. [174] NiO/NiAl O 9(9) H 600 1-2 TGA 2006 2 4 2 NiO, CuO, Mn O , Zafar et al. 2 3 [89] Fe O with SiO and 8(4) CH /H O/CO /N 800-1000 0.18-0.25 TGA t,y 2006 2 3 2 4 2 2 2 MgAl2O4 Abad et al. Natural gas, [85] Fe2O3/Al2O3 1(0) 800-950 0.09-0.212 FzB, CFzB h,q 2007 Syngas, CH4 Fe2O3/Al2O3, Abad et al, CH4 or CO or H2 [175] NiO/NiAl2O4, 3(0) 500-950 0.15-0.2 TGA u 2007 with H2O/CO2 CuO/Al2O3 H2/H2O/N2, Fe2O3/Al2O3, Abad et al., CO/CO2/N2, [176] NiO/NiAl2O4, 3(0) 450-1000 0.15-0.2 TGA f, u 2007 H2/CO/CO2/H2O, CuO/Al2O3 H2/CO/CO2 Berguerand & Lyngfelt, [80] Ilmenite 1(1) RSA Coal 950 0.09-0.25 CFzB z 2007 Berguerand & Lyngfelt, [81] Ilmenite 1(0) Pet Coke 950 0.09-0.25 CFzB z 2007 Corbella and 900, 0- [177] Fe O , Fe O /TiO 11(11) CH , CH in N m 0.2-0.4 FxB m Palacios, 2 3 2 3 2 4 4 2 1000 2007 de Diego et TGA 0.1-0.3, 0.2- [79] CuO/Al O 1(0) CH , CH /H O 700-800 CFzB, TGA r al.,2007 2 3 4 4 2 0.5 Erri & NiO/NiAl O , Varma, [178] 2 4 2(2) CH /H O/Ar 800-1200 0.21-0.43 TGA m NiO/Ni Mg Al O 4 2 2007 1-x x 2 4 Erri & NiO/NiAl O , CH /H O/Ar, Varma, [179] 2 4 7(5) 4 2 800-1200 0.21-0.43 TGA m NiO/Ni Mg Al O H /CO/CO 2007 1-x x 2 4 2 2 He et al., [180] Fe O /Al O 1(1) CH 900 0.1-0.3 TGA, FxB 2007 2 3 2 3 4

Hossain et Ni/Al2O3, Co- [181] 3(3) CH4 680 0.06-0.1 FzB al., 2007 Ni/Al2O3 m Hossain & [182] Ni/Al2O3, Co- 6(5) CH4, H2/Ar 650, 0- 0.07 FzB m, u

m de Lasa, Ni/Al2O3, Co/Al2O3 750 2007 Johansson NiO/MgAl O , M, et al [183] 2 4 2(0) CH /H O 950 0.125-0.18 FzB v NiO/NiAl O 4 2 2007 2 4 Leion et al. , Petcoke +H O/N [71] Fe O /MgAl O 1(0) 2 2 850-1000 0.09-0.125 FzB w, x 2007 2 3 2 4 Syngas Linderholm 0.132 [76] NiO/NiAl O 1(1) Natural gas 660-950 CFzB r et al., 2007 2 4 (=mean) CH /H O, 0.09-0.125 Mattisson et 4 2 [184] NiO/NiAl O 1(0) CH /H O/CO /N 750-950 0.125-0.18 TGA, FzB c,e al. 2007 2 4 4 2 2 2 (TGA) 0.18-0.25 TGA, Siriwardane NiO/Bentonite, NiO, Pressurized [185] 3(2) CO /CO/He/H 700-900 0.074-0.840 c, f et al., 2007 Bentonite 2 2 FxB, TEOM 0.09-0.125 Zafar et al., [186] NiO/MgAl O 1(0) CH /H O/N 800-1000 0.125-0.18 TGA c, u 2007 2 4 4 2 2 0.18-0.25 0.09-0.125 Zafar et al., [187] Mn O /Mg-ZrO 1(0) CH /H O/N 800-950 0.125-0.18 TGA c, u 2007 3 4 2 4 2 2 0.18-0.25

# O.C. (new) = number of oxygen carriers studied in g Cylindrical form, diameter×height the paper. The number in parenthesis indicates how h Data from continuous CLC reactor many of these that has not been studied before. i Spray dried particles. Oxygen carriers can vary by being prepared by j Natural iron ore. different authors, using different manufacturing k Study of reduction techniques, different raw material, different ratio of l Study of oxidation active/inert material and using different sintering m Temperature programmed reduction n temperature (or time). La0.8Sr0.2Co0.2Fe0.8O3 o Dp = particle diameter 50 kW Chemical-Looping Combustor TGA = Thermogravimetric analyzer p Study of de-fluidization FxB = Fixed bed q 300 W Chemical-Looping Combustor FzB = Fluidized bed r 10 kW Chemical-Looping Combustor CFzB = Circulating fluidized beds, i.e. chemical- s In Korean looping combustor t Chemical Looping reforming TEOM= Tapered element oscillating microbalance u Study on kinetics a In Japanese v Pulse experiment b w Effect of H2O on reduction/oxidation Study on sulfur c Effect of particle size on reduction/oxidation x Study on solid fuel d y No NOx verified in experiments In-situ XRD e Study of carbon deposition z 10 kW Chemical-Looping Combustor for use with f Effect of pressure solid fuels

As seen from the table above, more than 600 different oxygen carriers have been tested up to this point. It is difficult to make a direct comparison of all the results in the papers listed in Table 1-5. This is because reactivity data are very dependent upon oxygen carrier system, preparation method, fuel, reactor type, experimental procedure, particle size and reaction temperature. However, some general conclusions can be made from all these studies:

• Nickel oxides and copper oxides are by far the most reactive oxygen carrier materials • Copper oxides have a disadvantage of being apt to de-fluidize and agglomerate, although some researchers have prepared well-suited particles based on copper [78, 79, 156]

• Nickel oxides can not totally convert the fuel gases to CO2 and H2O. Besides, reduced Nio catalyzes steam reforming and carbon formation

• The reduction reactivity is faster with H2 and CO as a fuel than with CH4 • Reactivity generally increases with reaction temperature, although high reactivity has also been seen at relatively low temperatures in many cases • Effect of particle size has been given little attention and therefore, no real correlation between particle size and reactivity has been established

1.4.7. Oxygen Carriers in this study

To put the work performed in this thesis into perspective, the investigations presented in the preceding table should be kept in mind. In 2002, when the work presented here was initiated, only limited work had been performed with respect to oxygen carrier development. Hardly any results were obtained from fluidized beds, many investigations were performed with hydrogen as a fuel, very low temperatures were usually used in the experiments and the particle size was often larger than 1 mm. In addition to this, the number of tested oxygen carriers was limited and manganese oxides had hardly been tested and considered as an oxygen carrier. Since particle development was, and still is, the perhaps most crucial step for the overall success of chemical- looping combustion, it was decided that a large screening of suitable material should take place. To adjust the experiments and results to more reality-based conditions, several parameters were changed in comparison to earlier studies. This includes performing the reactivity tests in a fluidized bed, to use much smaller particle diameters, to use a higher temperature, 950°C, to use

CH4 and to also include manganese oxides as a possible oxygen carrier.

In the papers included in the appendix of this thesis all oxygen carriers but one have been prepared with a freeze-granulation method, which can be used to create spherical and highly porous particles. Although this specific method might not be commercially attractive due to the high manufacturing cost, it is believed that the particle behavior with respect to parameters such as the amount of inert and the sintering temperature will be highly relevant for other more commercial production methods. Also, the freeze granulation method has many similarities to commercial spray-drying, and it is believed that similar type of oxygen carriers can be produced using both methods.

Although different fuels are possible for a chemical-looping combustor, methane, CH4, was chosen as fuel for an initial screening of the oxygen carriers. This gas, which is the major 30 constituent in natural gas, was considered to be most relevant for the ranking of the particles. Moreover, a fast reaction with methane would probably mean a high reactivity with other gaseous fuels as well, although the ranking could come out different.

In the eleven papers presented in this thesis, four different metal oxide systems for the active material in the oxygen carriers have been investigated; Fe2O3/Fe3O4, Mn3O4/MnO, CuO/Cu and NiO/Ni. As added inert, several different materials have been used. These different inert materials can be divided into sub-groups; Al2O3 based material, stabilized and ordinary ZrO2,

TiO2, SiO2 and MgO. Different ratios of active/inert material and different sintering temperatures have been tested to optimize the carriers. A summary of all investigated iron-based oxygen- carriers is found in Table 1-6, manganese-based in Table 1-7, copper-based in Table 1-8 and nickel-based in Table 1-9. The tested carriers are presented as non-white squares. The numbers presented in the squares are used to identify the particles in the graphs in the results section. The Roman numbers indicate in which paper(s) the particles have been investigated. Also presented in the tables is the abbreviation of the particle, which together with the sintering temperature forms the nomenclature used in the papers. Hence, F4S950 corresponds to 40 wt% of Fe2O3 with SiO2 as inert sintered at 950°C.

Table 1-6 A summary of the iron-based oxygen-carriers Sintering temperature (°C) Oxygen carrier composition. (wt%) Abbr. 950 1100 1125 1150 1175 1200 1300 1400

40% Fe2O3/60% SiO2 F4S S S S

I,III I,III I,III 40% Fe2O3/60% ZrO2 F4Z 1 2 3

II, III II, III II, III 60% Fe2O3/40% ZrO2 F6Z 4 5 6

II, III II, III II, III 80% Fe2O3/20% ZrO2 F8Z 7 8 9

* * II,III 22% Fe2O3/78% Al2O3 F22A 10 11II, 12II, 13II, 60% Fe2O3/40% Al2O3 F6A III III, IX III 14II, 60% Fe2O3/40% Al2O3 F6A** III

60% Fe2O3/32% Al2O3 +8% F6AB 15I 16I 17I Bentonite 18II, 19II, 20II, 60% Fe2O3/32% Al2O3 +8% Kaolin F6AK III III III I, VI I, VI I,VI 40% Fe2O3/60% MgAl2O4 F4AM 21 22 23

I,II, 24II, 25 29I,II, 30II, I,VI I,VI I,VI 60% Fe2O3/40% MgAl2O4 F6AM III, VI, 26 27 28 III, VI III, VI III, VI X I,VI I,VI I,VI 80% Fe2O3/20% MgAl2O4 F8AM 31 32 33 34II, 35II, 36II, 40% Fe2O3/60% TiO2 F4T III III III 37II, 38II, 39II, 60% Fe2O3/40% TiO2 F6T III III III * Particle prepared by impregnation ** In Paper III this is denoted F6AB, however that name is avoided here to avoid confusion with the F6AB where “B” indicates bentonite. For preparation, different raw materials were used of both Fe2O3 and Al2O3 compared to particles 11-13. Additionally, this particle was only sintered for 4 hours. S Particles being too soft I, II… Investigated in paper I, II…

Table 1-7 A summary of the manganese-based oxygen-carriers Sintering temperature (°C) Oxygen carrier composition. (wt%) Abbr. 950 1100 1150 1200 1300

I 37% Mn3O4/60% SiO2 M37S* S 40 L L

I,V I,V I,V 37% Mn3O4/60% ZrO2 M37Z** 41 42 43

II II II 37% Mn3O4/60% ZrO2 M37Z* 44 45 46

II II 60% Mn3O4/40% ZrO2 M6Z 47 48

II II 78% Mn3O4/22% ZrO2 M78Z*** 49 50

I,V I,V I,V I,V 40% Mn3O4/60% Ca-ZrO2 M4CaZ 51 52 53 54

I,V I,V I, V, IX I,V I,V 40% Mn3O4/60% Mg-ZrO2 M4MgZ 55 56 57 58 59

I,V I,V I,V 40%Mn3O4/60% Ce-ZrO2 M4CeZ 60 61 62

II 26% MnO2/74% MgO M26M S 63

* Starting material was 40% MnO2 but as MnO2 decomposes to Mn3O4 during sintering, the active amount of oxygen carrier decreases to 37%. ** Initially prepared with 10% of graphite to create porous structure. Graphite was burned off during heat treatment. Starting material was 40% MnO2 but as MnO2 decomposes to Mn3O4 during sintering, the active amount of oxygen carrier decreases to 37%. *** Starting material was 80% MnO2 but as MnO2 decomposes to Mn3O4 during sintering, the active amount of oxygen carrier decreases to 78%. S Particles being too soft L Limited or no reactivity I, II… Investigated in paper I, II…

Table 1-8 A summary of the copper-based oxygen-carriers Sintering temperature (°C) Oxygen carrier composition. (wt%) Abbr. 950 1100

II 60% CuO/40% SiO2 C6S 64

II II 40% CuO/60% ZrO2 C4Z 65 66

II 40% CuO/60% TiO2 C4T 67 C C Sintered to cake during heat treatment II Investigated in paper II

Table 1-9 A summary of the nickel-based oxygen-carriers Sintering temperature (°C) Oxygen carrier composition. (wt%) Abbr. 950 1100 1200 1300 1400 1500 1600 68II, 69II, 70II, 40% NiO /60% ZrO2 N4Z IV IV IV 74II,

VII, VIII I,IV II,IV II,IV 40% NiO/60% NiAl2O4 N4AN* 71 72 73 XI

75II

I 40% NiO/60% NiAl2O4 N4AN** 76

77II, 78II, II,IV II,IV 40% NiO/48% NiAl2O4 +12% Kaolin N4AK* 79 80 IV IV

I 40% NiO/48% NiAl2O4 +12% CaO N4AC* 81 C

40% NiO/48% NiAl2O4 +12% N4AB* 82I Bentonite 83II, 84II, 85II, 86I,IX, I I 60% NiO/40% MgAl2O4 N6AM 87 88 IV IV IV X, XI 60% NiO/40% MgO N6M S S L 89II, 90II, 91II, 40% NiO/60% TiO2 N4T IV IV IV 92II, 93II, 94II, 80% NiO/20% TiO2 N8T IV IV IV

* Starting material was pure α-Al2O3. During heat treatment Al2O3 reacts with NiO to form NiAl2O4. ** Starting material was a technical grade α-Al2O3. During heat treatment Al2O3 reacts with NiO to form NiAl2O4

Particle 74 and 75 are from the same batch and sintered at the same temperature, although particle 75 was sintered for more than the normal 6 hours to examine the effect of the time used for sintering S Particles being too soft L Limited or no reactivity C Sintered to cake during heat treatment I, II… Investigated in paper I, II…

The eleven papers included in the appendix are not presented chronologically but rather logically, i.e. starting with papers which present a general overview of oxygen carrier performance and ending up with papers which present parameter evaluations of some oxygen carrier particles. Some of the oxygen carriers appear in more than one paper as can be seen in Table 1-6 to Table 1-9. Note that all papers, except IX & XI, deals only with chemical-looping combustion using methane as fuel which then naturally will be the main focus of this thesis. Below follows a short overview of the contents in each paper.

Paper I: Investigates 50 different oxygen-carriers based on nickel-, iron- and manganese oxide with different inert material Paper II: Investigates 58 different oxygen-carriers based on nickel-, iron-, copper- and manganese oxide with different inert material Paper III: Investigates 27 iron oxides with different inert material Paper IV: Investigates 19 nickel oxides with different inert material Paper V: Investigates 15 different manganese-based oxygen-carriers (40 wt%) with 60 wt% of stabilized or un-stabilized zirconia.

Paper VI: Investigates 15 different iron-based oxygen-carries with MgAl2O4, where the ratio of metal oxide to inert material and sintering temperature was optimized.

Paper VII: Investigates NiO/NiAl2O4 particles that had been used for more than 100 h in a 10 kW chemical-looping combustor. A comparison between fresh and used material was carried out.

Paper VIII: Further investigation of fresh NiO/NiAl2O4 particles sintered at 1600°C which were chosen as starting material for use in a 10 kW chemical-looping combustor. Paper IX: Reactivity investigation of promising oxygen carriers based on iron-, nickel- or manganese oxide with syngas. A comparison between syngas and methane as fuel was performed. Paper X: Testing of adding small amounts of nickel oxides to a bed of iron oxides to achieve a synergetic effect, where catalytic properties of nickel are utilized to convert methane to a more reactive syngas which can react with the iron oxide at an enhanced rate.

Paper XI: Comparison of two promising nickel oxides by use of pulse experiments. Information about reduction behavior for different chemical-looping technologies was obtained.

1.5. Objective

The aim of this work has been to develop, test and evaluate suitable oxygen carrier materials for chemical-looping technologies, mainly chemical-looping combustion. The working procedure has been the following:

Promising carriers should be selected based on results of reactivity tests as well as physical and chemical characterization. If possible, the characteristics of the oxygen carrier could be improved by changing parameters during preparation. The selected particles should also be further tested in a chemical-looping combustor/reformer in continuous operation and after such tests be investigated again to discover possible physical or chemical changes of the particles. If successful, the preparation method of suitable carriers should provide a basis for a recipe of larger batches of oxygen carriers prepared by other cheaper manufacturing methods. In addition, the reactivity data from the tests in the batch fluidized bed reactor would also provide information useful for a preliminary design of a real reactor.

The testing procedure was optimized to allow for a first comparison of a larger number of materials with highly variable reactivity and oxygen transfer capacity, R0. The amount of bed material was chosen as a compromise giving the best results for oxygen-carriers of intermediate reactivity. Detailed investigation of the reactivity of materials of low reactivity is of limited interest and for the most reactive and otherwise interesting materials, additional tests with smaller beds is possible.

The testing procedure should also been seen in the context of the EU/ECSC projects which funded most of this work, and in which Chalmers co-operated with CSIC (Consejo Superior de Investigaciones Científicas) in Spain on particle development. These projects involved a first screening where particles produced by extrusion were examined in TGA, a second screening in which particles were produced by freeze-granulation and investigated in laboratory fluidized bed at Chalmers, and a last phase where selected particles were examined for detailed kinetics at CSIC and tested in chemical-looping combustors at Chalmers.

2. Experimental

2.1. Method and purpose of investigation

A standard investigation method was developed in order to compare oxygen carriers for CLC in a consistent manner. This involves; testing of fresh material with respect to strength (crushing strength), physical appearance (light microscopy and scanning electron microscopy, SEM) and chemical structure (X-ray diffraction, XRD). The particles were then submitted to a day of simulated chemical-looping combustion in a laboratory fluidized bed. In these experiments the particles did not circulate between two reactors, instead oxidizing and reducing gases were alternately introduced to the bed for several cycles. In these tests, outgoing flow and gas concentrations were measured, which gives information about the reactivity of the particles. Most focus in this work has been on the reduction of the particles with the fuel. There are different reasons for this; firstly the oxidation is usually so fast initially that all oxygen is consumed and therefore the reaction rate is limited by the supply of oxygen. Thus a realistic rate is not possible to determine with the current experimental set-up and procedure. And secondly, oxidation only consumes oxygen and full conversion of all the oxygen is not needed, whereas the reduction is of greater interest because of the importance of high conversion of the fuel to carbon dioxide and steam. The reactions in the fuel reactor are also more complex with possible side reactions, like carbon formation and reforming.

In most experiments, pressure drop measurements over the bed showed if the bed was fluidized or not, and thus it was possible to determine when possible agglomerations were formed. The experiments were terminated after a reducing period and a new physical and chemical investigation of the particles was performed. If a second experiment of a sample was done, the experiment would then be terminated after an oxidizing period and followed by a new physical and chemical investigation.

If a promising particle or group of particles were found, a narrower region of parameters could be altered to improve the particles. This includes using slightly different raw materials, using different ratios of metal oxide and inert or using different sintering temperatures. For such promising oxygen carriers, a more detailed investigation was often carried out, where reactivity of

37 the oxygen carrier was investigated using different size ranges, temperatures or experimental procedures.

2.2. Preparation of oxygen carriers

All particles but one were prepared by a freeze granulation method. Here, a water-based slurry of the raw material, in the form of fine chemical powders, was prepared by ball milling for 24 h. A small amount of dispergant was also added to this mixture in order to improve slurry characteristics. After milling, an organic binder was added to the slurry as a binder to keep the particles intact during later stages in the production process, i.e. freeze-drying and sintering. Spherical particles were produced by freeze-granulation, i.e. the slurry is pumped to a spray nozzle where passing atomising-air produce drops, which are sprayed into liquid nitrogen where they freeze instantaneously. The frozen water in the resulting particles is then removed by sublimation in a freeze-drier operating at a pressure that corresponds to the vapour pressure over ice at -10ºC. After drying, the particles were sintered at temperatures between 950ºC and 1600ºC for 6 h using a heating rate of 5ºC/min. Finally they were sieved to obtain particles of well- defined sizes.

The last particle, F22A1100 - # 10 in Table 1-6, was prepared by an impregnation method. The preparation method for this is described in Paper III. Figure 2-1 shows a light microscope image and two SEM images of a freeze granulated particle, F4AM1100 (from Paper VI). As seen, the particles are spherical. Almost all particles had this spherical shape, although the surface structure could vary considerably depending upon the material and sintering temperature. For a comparison of the surface structures of all oxygen carriers tested, see the appendix (chapter 8).

. a) b) c)

Figure 2-1. Freeze granulated particles of 40% Fe2O3/ 60% MgAl2O4 sintered at 1100ºC (particle #21). a) Light microscope image b) SEM image of particle and c) SEM image of surface.

2.3. Characterization of fresh and reacted oxygen carriers

In order to determine possible chemical transformations that occurred in the samples during both the sintering process as well as the reactivity investigation, all of the fresh and reacted samples were characterized using x-ray powder diffraction (Siemens D5000 Powder Diffractometer utilizing Cu Kα radiation). See the appendix for an overview of the XRD data for the oxygen carriers used in this thesis. Also the shape and morphology of both fresh and reacted oxygen-carrier particles were studied using a light microscope as well as an analytical scanning electron microscope (Electroscan 2020).

The force needed to fracture the particles was measured using a Shimpo FGN-5 crushing strength apparatus. The crushing strength was measured on particles in the size range 0.180-0.250 mm and taken as the average value of 30 results. Crushing a particle is what Bemrose and Bridgwater calls a “single particle test”.[188] It gives information about the strength of the particle itself but not necessarily a full understanding of the survival of the particle in a full size circulating unit where several other parameters, such as other particles, temperature, velocities, shape, pressure, chemical reactions etc, may affect the attrition rate. Therefore, a lower limit of an acceptable crushing strength can not be defined from these tests. Thus, crushing strength has to be supplemented with tests in a larger circulating unit at relevant temperatures and velocities and with chemical reactions taking place. Nevertheless, in this work, it is assumed that harder particles are better than softer ones from a lifetime perspective.

The density of the tested particles is measured through a simple test. 5 ml of particles are filled in a cylinder and its weight is measured. The density is then simply calculated using these numbers using a void factor of 0.37, which accounts for the assumed voidage in the cylinder caused by the space between the spherical particles.[189] This density is here called apparent density.

2.4. Reactivity Investigation

The laboratory fluidized-bed reactor system as used in the experiments can be seen in Figure 2-2. The procedure of the reactivity investigation in the laboratory fluidized-bed reactor has been described in Paper III.

Figure 2-2 Experimental setup for the laboratory fluidized bed reactor

The important experimental parameters for the screening tests of the oxygen carriers are summarized in Table 2-1.

Table 2-1 Summary of experimental parameters for screening of oxygen carriers Experimental data Reactor data Mass of bed (g) 10 or 15 Material Quartz Size interval particles (μm) 125-180 (90-125, 180-250) Height (mm) 820 Height bed (mm) 4-33 Width of porous plate (mm) 30 Apparent density, particles 800-4500 Diameter under plate (mm) 19 (kg/m3) # of cycles in experiment 4-26 Diameter above plate (mm) 30 Temperature reactor (°C) 950 (down to 650) Measuring Data Pressure (bar) 1 Temperature measurement 10% Pt/Rh Honeywell u/umf* 1.8-12 Pressure measurement transducers Frequency of pressure drop meas. Reducing gas 50% CH + 50% H O 20 4 2 (Hz) Rosemount, NGA Oxidizing gas 5% O in N Gas analyzer 2 2 2000 CH4, CO2, CO and Inert gas 100% N2 Analyzed gases out O2 Flow in reduction (mlN/min) 600 or 900 (176 or 264 W) Logging interval (s) 2 Flow in oxidation (mlN/min) 600 or 1000 Other logged info Flow, time, ΔP, T Flow in inert (mlN/min) 600 or 900 Time in reduction Until fully reduced** Time in oxidation Until fully oxidized Time in inert (s) 180 or more * Theoretical fluidizing velocities calculated from Kunii and Levenspiel[189]

** Fully reduced means Fe3O4, MnO, Cu and Ni for the corresponding oxygen carriers

The initial screening experiments were performed in a laboratory fluidized-bed reactor of quartz at a temperature of 950°C (850°C for copper oxides). Methane was chosen as fuel. Steam was added to the fuel in order to avoid possible carbon formation. An experiment was typically performed during one working day. The experiments were conducted in cycles; with the oxygen carrier sample exposed to alternating periods of oxidation-inert-reduction-inert. The inert period consists of pure nitrogen, the purpose being to avoid mixing of the reactive gases. Time, reaction

40 temperature (measured 5 mm below and 38 mm above the bed plate), pressure difference over the bed, outgoing flow and the concentrations of CH4, CO2, CO and O2 were logged. An example of a full cycle can be found in Figure 2-3.

Two different bed masses (10 & 15 g) and two different fuel-flows (300 & 450 mLN/min) have been used in the screening tests. However, a lower bed mass has been accompanied by a lower flow and vice versa. In this manner the experiments have been comparable and the solid gas inventory has been kept steady at 56.8 kg oxygen carriers/MWCH4.

Figure 2-3 An example of concentration profiles (dry basis) of a full cycle for F4AM1100 (particle #21). CH4 ("), CO2 (M), CO ((), H2 (*) and O2 (.), X (--) For definition of X, see equation 4. The vertical dashed lines indicate at which time the gases are switched. Gas concentration during reduction period will be shown with better resolution in the subsequent figure.

Figure 2-4 to Figure 2-7 show the outlet concentration of gaseous components for one reduction and one oxidation period conducted with one type of oxygen carrier for each of the four metal oxide systems investigated, i.e. iron-, manganese-, copper- and nickel oxide. For the reducing period, the graph starts when the reducing gas was introduced and a delay of approximately 20-30 seconds is seen before the gas reaches the analyzers. The vertical dashed lines indicate the time when the inert gas is introduced. Also this change of gas is followed by a delay of approximately 20-30 s before all gases from the reduction period are replaced by nitrogen.

During the reductions in Figure 2-4a to Figure 2-7a, the incoming methane initially reacts almost completely to form CO2 and H2O for all particles. For nickel oxides there is a thermodynamic limitation [90, 91], and hence some CO and H2 are at all times present during the reduction. For copper oxide, Figure 2-6a, there is an initial methane peak that later diminishes in the period. To a smaller extent this is also true for most of the nickel oxides particles investigated, although not visible in Figure 2-7a. As the reduction proceeds and the conversion to CO2 gets lower, the different types of metal oxides display different types of behavior. For iron, manganese and copper a large proportion of the methane passes unreacted through the bed and there is formation of some minor amounts of CO and H2. H2 is not measured on-line, but is assumed to be related to the outlet partial pressure of CO and CO2 through an empirical relation based on the equilibrium of the gas-shift reaction. Since it was not measured on-line, it is not presented in the figures. For the nickel oxide, methane does not pass through the reactor unreacted, but reacts to CO and H2, most likely through methane pyrolysis or steam reforming (15, 16). Formation of carbon can also be detected for some nickel-, copper- and manganese-oxides. This is detected as

CO and/or CO2 from the outlet of the reactor during both the inert and oxidizing periods, and an example of this is found in Figure 2-6b.

As for the oxidation periods in Figure 2-4b to Figure 2-7b, full conversion of the incoming oxygen occurs initially. This means that the oxidation is fast and limited by the supply of oxygen. The reason for conducting the experiments with a relatively low concentration of oxygen instead of air is to avoid too high temperatures due to the exothermic oxidation. Note that for the oxidation of the copper oxide, in Figure 2-6b, there is a discontinuity in the reactivity of O2 about 1500 s into the oxidation period. This may be associated with the fact that the oxygen partial pressure needs to be above approximately 0.5 % for CuO to be stable at 850°C.[90] The peak in the discontinuity is not well understood but could possibly be associated with an induction period in the onset of oxidation of Cu2O to CuO or it could be a temperature phenomenon; the reaction is exothermic and when it slows down it could give a temperature decrease that affects the equilibrium pressure. So during the oxidation, Cu is first oxidized to Cu2O and when the oxygen partial pressure is high enough, oxidation to CuO starts. The four oxygen carriers presented in Figure 2-4 to Figure 2-7 are among the most reactive for each group of metal oxides investigated here. However, the concentration profiles presented here are fairly representative for their corresponding metal oxide system.

a) b) Figure 2-4. Concentration profiles (dry basis) of a) reduction b) oxidation of F4AM1100 (particle # 21). Vertical dashed lines indicate transition to inert gas.

a) b)

Figure 2-5. Concentration profiles (dry basis) of a) reduction b) oxidation of M78Z950 (particle # 49). Vertical dashed lines indicate transition to inert gas.

a) b)

Figure 2-6. Concentration profiles (dry basis) of a) reduction b) oxidation of C6S1100 (particle # 64). Vertical dashed lines indicate transition to inert gas.

a) b)

Figure 2-7. Concentration profiles (dry basis) of a) reduction b) oxidation of N6AM1400 (particle # 86). Vertical dashed lines indicate transition to inert gas.

The gas flow is measured in connection with the gas analysis, and flow variations upstream of the analyzer will rapidly affect the measured flow, whereas changes in gas concentrations have to reach the analyzer before being recorded. Hence, the flow and concentration profiles in Figure 2-4 to Figure 2-7 are affected both by the delay for the gas concentration and by changes in gas flow due to reaction as well as condensation of steam in the condenser. However, almost all

44 dispersion in the laboratory equipment is expected to take place after the bed of particles, to some extent above the bed but mainly in the condenser after the reactor, due to its relatively large size and low temperature.

The flow behaviour seen in Figure 2-7a can be explained as follows; A gas mixture of 50% CH4 and 50% steam is obtained by letting pure methane flow through a temperature controlled humidifier set at a temperature to give 50% steam. The first decrease in flow seen is most likely associated with difficulties keeping a stable concentration of steam in connection with the switching of flows. The subsequent increase in flow is explained by oxidation of CH4, whereby each molecule of CH4 gives one CO2 and two H2O, thus resulting in a volume expansion and flow increase. As the steam added with the incoming methane, as well as that produced by the reaction in the fluidized bed, reaches the condenser, there will be a flow decrease due to steam condensation. After this decrease follows a period of rather stable flow of > 450 mLn/min. This flow consists mainly of CO2, CO and H2 and is slightly higher compared to the incoming methane. The higher flow may be due to some H2 formation, which was not measured in this experiment. After this follows a period of incomplete oxidation of CH4, giving one CO and two

H2. This increase in flow is a gradual phenomenon and is caused by the continual decrease in reactivity of the oxygen carrier. There is an additional strong increase in flow when the gas is switched back to N2. The flow coming out after this switch is significantly higher than the incoming flow because CO is produced by oxidation of coal on the particles.

The flow measurement device was calibrated in pure nitrogen and although not perfectly linear, gas molecules of similar or higher molecular weights (as CO2, CO, O2) give essentially correct flows. There is a negligible underestimation of methane flows whereas the small hydrogen molecule clearly gives an underestimated flow. Therefore a slight error in the mass balances occurs when there are large proportions of hydrogen passing through the reactor, as may take place in the later stages of reduction when the degree of oxidation is low and reforming and net carbon formation may occur. This was verified by making mass balances of ingoing and outgoing carbon over a reducing period; if the gas yield is high, the mass balances are essentially correct.

From measured flow and outlet gas concentrations the reactivity of the particles can be calculated. All the necessary calculations are explained in Paper III, although some of the equations are presented below. In almost all cases the third reduction period is chosen for the calculations. The reason for this is that many experiments show lower reactivity during the first

45 reduction period, after which the rate stabilizes and becomes constant with very similar concentration profiles from the third reduction period and onwards.

The degree of oxidation, or conversion, is defined as;

m − m X = red (4) mox − mred

where m is the actual mass of sample, mox is the mass of the sample when fully oxidized and mred the mass of the sample in the fully reduced form. In order to facilitate a comparison between different oxygen carriers that contain varying amounts of oxygen depending upon the fraction of inert, a mass-based conversion was defined as;

m ω = =1+ Ro (X −1) (5) mox

where Ro is the oxygen ratio, defined as:

Ro= (mox − mred ) / mox (6)

The oxygen ratio is the maximum mass fraction of the oxygen-carrier that can be used in the oxygen transfer, and is dependent on the metal oxide used as oxygen carrier as well as the amount of inert in the particles.

To facilitate a comparison of reaction rates between different oxygen carriers a rate index was defined as the normalized rate, expressed in %/min;

Rate index = 60*100*(dω/dt)norm, (7)

-1 where (dω/dt)norm is the normalized average rate expressed in s , and calculated from

dω ( ) = k ⋅ p (8) dt norm eff ref

46 where pref is a reference partial pressure of methane, here chosen to be 0.15, which would approximately correspond to a full conversion of the gas, see discussion below. If the mass transfer resistance between the bubble and emulsion phases in the fluidized bed reactor is small, and assuming that the reaction between the methane and solid is first order with respect to methane, the exposure of particles to methane can be represented by a log-mean partial pressure of methane, pm, which can be defined in terms of the inlet and outlet partial pressure:

p − p p = CH 4,in CH 4,out (9) m p ln( CH 4,in ) pCH 4,out

Assuming an inlet partial pressure of methane of 1 and an outlet partial pressure of 0.001 the log- mean partial pressure would be 0.145, cf eq. 9.

The effective first order reaction rate constant, keff in eq. 8, was calculated from

1 dω keff = (10) pm dt

For calculations of the rate index and the normalized rate of reaction in equation (7) and (8) the average rate constant in the interval of Δω=0.01 where the reactivity is the highest was chosen.

For conversion of the gas, three different gas yields are defined depending on type of fuel:

p γ = CO2 (11) CH 4 ( p + p + p ) CH 4 CO2 CO

p γ = CO2 (12) CO ( p + p ) CO2 CO

p γ = H 2O (13) H 2 ( p + p ) H 2 H 2O

In order to evaluate oxygen carriers suitable for CLR(a) the stochiometric ratio of the gases leaving the reactor during the fuel addition phase, λF, is defined as:

(2 p + p + 2( p + p ) − p ) λ = CO2 ,out CO,out CO2 ,out CO,out H 2 ,out (14) F 4( p + p + p ) CH 4 ,out CO2 ,out CO,out

Here, the denominator corresponds to the amount of oxygen that is needed to fully convert the incoming methane to CO2 and H2O and the numerator corresponds to the amount of oxygen that has been supplied by the oxide in the outgoing gas. Hence a λF of 1 equals full conversion of the bed to CO2 and H2O and a lower λF would mean that also CO and H2 would be present. It has been suggested that a λF of 0.3-0.4 is suitable for CLR(a), providing that the conversion of methane is almost complete.[58]

In this thesis the results from the reactivity experiments from the screening tests for CLC will be presented as Rate Index vs. Crushing Strength together with information of which particles that de- fluidized during the experiments. Rate index was introduced as a measure of the reactivity in order to be able to easily compare different oxygen carriers. Crushing Strength is the force needed to crush a particle and is expressed in N. If a particle de-fluidized it was noted as a decrease in the pressure fluctuations over the bed. De-fluidization does not necessarily mean that a hard agglomerate was formed in the bed, in most cases only a small force was necessary to separate the particles from each other. Hence in the graphs presented in Results, properties of an oxygen carrier considered to be important will be represented; reactivity with methane (rate index), strength and if the particles de-fluidized or not. For a more detailed discussion on other important factors as gas yields and chemical characterization, the reader is referred to the papers in the appendix.

2.4.1. Pulse experiments

Pulse experiments is an alternative method for testing the reactivity of oxygen carriers, and is especially of interest for investigation of oxygen carriers suitable for CLR(a). The experimental set-up is the same as for normal experiments, as described above. However, in the pulse experiments the reduction period is divided into several shorter reduction periods, of 2 to 6 seconds each, with nitrogen introduced between each pulse. The advantage with this procedure is 48 that it is easier to follow how the reduction proceeds as the degree of conversion of the oxygen carriers is decreasing. In-between each reduction pulse, nitrogen is passing through the bed. A full cycle from a pulse experiment can be seen in Figure 2-8

Figure 2-8 An example of a concentration profile (dry basis) for a cycle of N6AM1400 (particle # 86) with a reduction consisting of a pulse experiment of 10 pulses of 6 s. Note that nitrogen is introduced for one minute between each pulse. CH4 ("), CO2 (M), CO ((), H2 (*) and O2 (.), X (--). Gas concentrations during the reduction period will be shown with better resolution in the subsequent figure.

From the pulse experiments, information on the gas selectivity, for each pulse, can be presented, as seen in Figure 2-9. Information gained for these concentration profiles will be presented in section 3.6.

a) b)

Figure 2-9 Concentration profiles for two pulse experiments with 10 cycles of 6 s (dry basis) of a) N6AM1400 (particle # 86) and b) N4AN1600 (particle # 74). Note that the sum of the gas concentrations do not add up to 100% which is due to dilution with nitrogen from inert periods.

3. Results and discussions

3.1. The influence of the sintering temperature

During preparation of oxygen carrier material, the sintering temperature is used to create hard and durable particles and in some cases (as with NiO on Al2O3) also to promote creation of a new inert material (NiAl2O4). Although the relation between the crushing strength of an oxygen carrier and physical survival time in a fluidizing bed reactor is not clearly established, a higher crushing strength is assumed to be an advantage. Therefore, the aim is to create particles that are not too soft and do not fragmentize too easily. A way of changing the strength and porosity of the oxygen carrier is to adjust the sintering temperature. A higher sintering temperature generally means that the oxygen carrier “shrinks” due to breakdown of the internal pore structure. This also results in an increase of the apparent density and the crushing strength as well as decrease in the reactivity due to the diminished porosity.

10 10 ength (N) r ushing st

r 1 1 C Crushing strength (N)

900 1000 1100 1200 1300 1000 2000 3000 4000 Sintering Temperature (C) . Density (kg/m3) a) b) Figure 3-1 Crushing strength for some manganese-based oxygen-carriers as a function of a) sintering temperature and b) apparent density. M4Z (), M4CaZ (+), M4MgZ (() and M4CeZ (.)

An example of how the sintering temperature affects the oxygen carriers is shown in Figure 3-1. This example, taken from Paper V, shows the correlation between crushing strength and sintering temperature and crushing strength and apparent density for manganese based oxygen carriers on stabilized and un-stabilized zirconia. With a few exceptions, these correlations are valid for all other oxygen carriers investigated here. A summary of how the crushing strength and

51 the apparent density varies as the sintering temperature increases can be found in Table 3-1 to Table 3-4.

Table 3-1 Crushing strength (top, in N) and apparent density (bottom, in kg/m3) for the iron-based oxygen-carriers Sintering temperature (°C)

Oxygen carrier composition. (wt%) Abbr. 950 1100 1125 1150 1175 1200 1300 1400

40% Fe2O3/60% SiO2 F4S S S S

2.6 5.3 5.9 40% Fe2O3/60% ZrO2 F4Z 2700 4069 3964 1.2 2 0.8 60% Fe2O3/40% ZrO2 F6Z 3466 4208 4022 0.6 1.9 1.4 80% Fe2O3/20% ZrO2 F8Z 1650 2682 3754

1.6 22% Fe2O3/78% Al2O3 F22A 2501 0.4 1.0 9.4 60% Fe2O3/40% Al2O3 F6A 1326 1993 3052 5.6 60% Fe2O3/40% Al2O3 F6A 3257 2.0 9.0 11.6 60% Fe2O3/32% Al2O3 +8% F6AB Bentonite 2135 2969 2914 0.6 2.7 7.2 60% Fe2O3/32% Al2O3 +8% Kaolin F6AK 1608 2385 3032 0.7 4.2 7.5 40% Fe2O3/60% MgAl2O4 F4AM 1458 2593 2865 0.7 1.8 2.3 3.9 6.8 11.8 12.5 60% Fe2O3/40% MgAl2O4 F6AM* 1318 2224 2371 2977 3436 3321 3207 7.5 12.7 7.3 80% Fe2O3/20% MgAl2O4 F8AM 3357 3542 3286 0.7 1.4 1.2 40% Fe2O3/60% TiO2 F4T 1108 2090 2388 0.6 2.5 1.6 60% Fe2O3/40% TiO2 F6T 1379 2979 3263 *For F6AM1100 and F6AM1200 two different prepared oxygen carriers with different strength and porosity exists. Information about this can be found in Paper VI.

Table 3-2 Crushing strength (top, in N) and apparent density (bottom, in kg/m3) for the manganese-based oxygen-carriers Sintering temperature (°C)

Oxygen carrier composition. (wt%) Abbr. 950 1100 1150 1200 1300

1.4 37% Mn3O4/60% SiO2 M37S S L L 1220 0.6 1.9 2.8 37% Mn3O4/60% ZrO2 M37Z 1673 2489 3047 0.4 0.6 7.7 37% Mn3O4/60% ZrO2 M37Z 1571 1752 4077 0.6 0.8 60% Mn3O4/40% ZrO2 M6Z 2571 2684 0.3 0.3 78% Mn3O4/22% ZrO2 M78Z 1439 1672 0.5 0.8 3.0 6.8 40% Mn3O4/60% Ca-ZrO2 M4CaZ 1823 2086 2963 4041 0.3 0.5 0.7 2.4 3.6 40% Mn3O4/60% Mg-ZrO2 M4MgZ 1628 1957 2264 3305 3691 0.7 1.8 11.3 40%Mn3O4/60% Ce-ZrO2 M4CeZ 1841 2712 4429 0.4 26% MnO2/74% MgO M26M S 818

Table 3-3 Crushing strength (top, in N) and apparent density (bottom, in kg/m3) for the copper-based oxygen-carriers

Sintering temperature (°C)

Oxygen carrier composition. (wt%) Abbr. 950 1100

2.6 60% CuO/40% SiO2 C6S 1241 0.8 2.7 40% CuO/60% ZrO2 C4Z 2142 3699 4.5 40% CuO/60% TiO2 C4T C 3157

Table 3-4 Crushing strength (top, in N) and apparent density (bottom, in kg/m3) for the nickel-based oxygen-carriers Sintering temperature (°C)

Oxygen carrier composition. (wt%) Abbr. 950 1100 1200 1300 1400 1500 1600

0.4 0.4 0.8 40% NiO /60% ZrO2 N4Z 1816 1925 3024 2.4 0.8 0.5 1.1 3554 40% NiO/60% NiAl2O4 N4AN 1588 1878 2602 3.8 3851

1.3 40% NiO/60% NiAl2O4 N4AN 2896 0.3 0.3 0.4 1.1 40% NiO/48% NiAl2O4 +12% Kaolin N4AK 1384 1259 1319 2040 3.2 40% NiO/48% NiAl2O4 +12% CaO N4AC C 2792 0.1 40% NiO/48% NiAl2O4 +12% N4AB Bentonite 805 0.2 0.2 0.9 2.2 3.4 7.0 60% NiO/40% MgAl2O4 N6AM 1278 1157 1452 3002 3654 4149 60% NiO/40% MgO N6M S S L

0.5 0.6 1.2 40% NiO/60% TiO2 N4T 1097 1727 3337 0.4 0.6 2.1 80% NiO/20% TiO2 N8T 1620 2378 3724

For comparison of crushing strength and survival in circulating units, several particles manufactured at Chalmers have been tested in 300 W and 10 kW chemical-looping combustors. The particles tested and the losses of fines collected from the experiments are shown in Table 3-5. Here, fines are defined as particles smaller than 45 μm. The results in the two units are not comparable as there is a very large difference in gas velocity.

As seen, although the strength of a few of the particles tested has been rather low, none of the experiments performed in the circulating units has resulted in a large loss of fines. Losses of particles larger than 45 μm is believed to be caused by inadequate design, e.g. of the cyclone, and not attrition, and should therefore not be seen as a problem related to the particle properties. An example of the attrition rate of fines as a function of time can be found in Paper VII.

Table 3-5 Tested freeze granulated oxygen carriers in Chalmers’ chemical-looping combustors Time of Loss of fines Tested Oxygen Crushing Ref. CLC/Circ. at end of Other carrier/ unit (kW) strength (N) (hours) operation Larger loss in N4AN1600/10 [73, 74] 2.4 100/300 0.0023 %/hour the beginning F6AM1100/10 [74] 1.8 17 - No significant N4AN1600/0.3 [82] 2.4 8/18 losses No significant N6AM1400/0.3 [82, 83] 2.2 30/150 losses Larger loss in M4MZ1150/0.3 [84] 0.7 70/130 0.038 %/hour the beginning F6A1100/0.3 [85] 1.4 40/60 No losses Formation of No losses of N6AM1400/0.3 [86] 2.2 41 small soft fines lumps

3.2. Results from different metal oxides using methane as fuel

In order to get an overview of all the different oxygen carriers investigated, a short section follows where the main results for each of the four different types of metal oxides - iron, manganese, copper and nickel - are presented separately. Note that all figures and conclusions are based on reactivity with CH4 unless otherwise stated.

3.2.1. Iron based oxygen carriers

The iron oxides particles are generally of high strength but low reactivity. As seen in Table 1-6 the particles prepared with SiO2 as inert material were too soft to use. The results from the experiment with iron oxides can be seen in Figure 3-2. As seen among the particles that de- fluidized, most are of high strength and low reactivity. The most promising group of particles are

Fe2O3 on MgAl2O4, notably those with 60 wt% metal oxide or less and sintered at a temperature lower than 1150°C. In the graph these are #21 and #24-26. This subgroup of particles was therefore selected as promising from a general study on iron oxides, presented in Paper III, and investigated more in detail in Paper VI. The purpose of Paper VI was partly to optimize the sintering temperature used for the particles containing 60% of Fe2O3. Note the large difference in strength and reactivity for the particle #25 sintered at 1100°C and particle #29 sintered at 1200°C. The temperature interval was studied in Δ25°C interval (particles #25-29), after which the conclusion was that particle 25 was the most promising, although the crushing strength was

55 somewhat low. The results from Paper VI led to the manufacturing of a large batch of F6AM1100 which was investigated for a short period in a 10 kW prototype reactor.[74] The same particle was also used in testing of mixed oxides, see section 3.2.5. Apart from mixing iron with

MgAl2O4 also ordinary Al2O3 led to particles with high reactivity when sintered at 1100°C or lower (#11, 12). F6A1100 was also produced in larger quantities and was successfully tested by Abad et al in the 300 W chemical-looping combustor at Chalmers. [85]

Figure 3-2 Rate index vs. crushing strength for iron-based oxygen-carriers. Circles around numbers indicate de-fluidization. Numbers are from Table 1-6.

3.2.2. Manganese based oxygen carriers

The manganese-based oxygen-carriers are often more reactive and less hard than the iron-based oxygen-carriers. As with iron, mixing manganese oxides with SiO2 led to failure during preparation or negligible reactivity as can be seen in Table 1-7. The poor reactivity is believed to be caused by formation of irreversible silicates during heat treatment, which also was noticed by Zafar et al.[88] Furthermore, M26M950 was too soft for any reactor testing. The results from the experiments with manganese based oxygen carriers can be seen in Figure 3-3. In contrast to the iron-based oxygen-carriers, de-fluidization for manganese is more common for the particles with high reactivity. Clearly, stabilized-zirconia and sintering temperatures of 1150°C or lower, result 56 in the highest reactivity of the particles produced (#51, 52, 55-57, 60 & 61). Despite the fact that M4MgZ1150, particle #57, de-fluidized, it was chosen and manufactured for further testing in the 300 W chemical-looping combustor.[84] This was because Cho et al performed additional tests on the same particle in a smaller reactor of 22 mm in diameter and found no signs of de- fluidization.[162] This finding suggests that the testing in the 30 mm laboratory reactor may overestimate the risk of de-fluidization for manganese-based oxygen-carriers which will be discussed in section 3.7. The tests performed in the 300 W reactor with this oxygen carrier were successful, with no signs of agglomeration or deactivation.[84]

Figure 3-3 Rate index vs. crushing strength for manganese-based oxygen-carriers. Circles around numbers indicate de-fluidization. Numbers are from Table 1-7.

3.2.3. Copper based oxygen carriers

The copper-based oxygen-carrier C4T1100 sintered to a cake during heat treatment. Due to the low melting temperature of Cu0 the experiments with copper were performed at 850°C. Despite that, 3 out of 4 particles tested de-fluidized in the reactor system, the exception being C4Z950. This can be seen in Figure 3-4. Due to the unsatisfactory results of these first copper oxides prepared, less focus was put on freeze-granulated copper oxides for use in chemical-looping combustion. 57

Figure 3-4 Rate index vs. crushing strength for copper-based oxygen-carriers. Circles around numbers indicate de-fluidization. Numbers are from Table 1-8.

3.2.4. Nickel based oxygen carriers

In general nickel oxides are superior to iron-, manganese-, and copper-oxides when it comes to reactivity with methane, which can be seen in Figure 3-5. Furthermore, the rate index presented here for the most reactive nickel oxides is an underestimation. This is because the rate is limited by the fuel flow and all incoming methane is transformed. More discussion concerning this can be found in Paper IV. Several very reactive and sufficiently hard particles exist, among which the best are N6AM sintered between 1300 and 1500°C (#85 - 87), N4AN sintered between 1300 and 1600°C (#71 - #76) and N4Z1300 (#70). N6AM1400 has been produced in a large batch and tested successfully in a 300 W reactor for both CLC and CLR(a).[82, 83, 86] It has also been successfully tested in a mixed oxide experiment which will be discussed in the following section. N4AN1600 was used in the first successful testing of more than 100 hours in a 10 kW chemical- looping combustor at Chalmers.[73, 74] In a subsequent study, Paper VII, the used particles from the 10 kW reactor were compared with fresh ones and the conclusion was that no major changes in reactivity, morphology or chemical composition had occurred during testing. This stability indicates that the particle could be suitable for use in chemical-looping combustion. Also, long time testing of N4AN1600 in a batch fluidized bed reactor in Paper VIII further establishes the

58 suitability of the particle. N4AN1600 has also successfully been tested with natural gas by Johansson et al in the 300 W reactor system at Chalmers.[82]

Figure 3-5 Rate index vs. crushing strength for nickel-based oxygen-carriers. Circles around numbers indicate de-fluidization. Numbers are from Table 1-9.

In Paper XI, N6AM1400 and N4AN1600 were compared using pulse experiments (using short intervals of reduction) to test the suitability for CLC and CLR(a & s). The results indicate that N6AM1400 was both better in converting methane and suppressing carbon formation at elevated temperatures, see section 3.6.

From Table 1-9 it can be seen that four of the nickel based oxygen carriers failed in production. For N4AC, a sintering temperature of 1600°C was too high, which resulted in a melted structure of the particles. All nickel oxides with MgO failed either due to lack of strength or negligible reactivity. The lack of reactivity was most likely due to formation of inert MgNiO2 during heat treatment.

3.2.5. Mixed oxide systems

The concept of using two different types of oxygen carriers simultaneously, i.e. mixed oxides, in order to achieve synergy effects was investigated in Paper X. The idea behind this concept came as a result of the findings for some individual oxygen carrier systems. In Paper VII and VIII it was shown that very small amounts of nickel was needed in order to fully convert the incoming methane in the batch fluidized bed reactor. This is because Ni0 is known to catalyze methane conversion, either through methane pyrolysis:

CH4 → C +2H2 (15) or steam reforming:

CH4 + H2O→ CO + 3H2 (16)

Furthermore, it is shown in Paper IX that iron (and manganese) is a lot more reactive towards syngas in comparison to methane (see section 3.5). Hence, the idea was to use a small amount of nickel oxides in a bed of iron oxides in order to increase the reactivity of the iron-oxide bed. Here, the small amount of nickel-based oxygen-carriers will act as a catalyst to obtain a reactive gas, mainly of CO and H2, which will react with the bulk iron oxide carriers. For these experiments, two highly reactive oxides were tested; N6AM1400 (#86) & F6AM1100 (#25). Several different ratios of nickel to iron in the bed were tested. The outcome can be seen in Figure 3-6, where the methane gas yield is presented as a function of the mass based conversion of the bed material. Here it is shown how only small additions of nickel greatly increases the conversion of methane.

Figure 3-6 Experiments performed with different ratios of N6AM1400 with F6AM1100. The methane yield, γCH4, is shown as a function of the mass based conversion rate, ω. All tests performed with 15 g bed with: 0% N6AM1400 (,), 1% N6AM1400 (>), 3% N6AM1400 (/), 5% N6AM1400 (.), 10% N6AM1400 (*), 50% N6AM1400 (() and 100% N6AM1400 (")

An example of what can be gained by using mixed oxides was shown for an experiment using 3% of nickel oxides to a bed of iron oxides, see Figure 3-7. Here it is demonstrated that a mixed oxide bed can produce roughly two times as much CO2 under a period of time, compared to that produced by the two metal oxides separately. This is explained by the synergetic effect that comes from using nickel to convert the methane to a syngas mixture which reacts much more quickly with iron oxide than methane.

Figure 3-7. Accumulated volume of CO2 as a function of time in reduction for: 100% F6AM1100 (14.55g) (,), 3% N6AM1400 (0.45g) with 97% F6AM1100 (14.55g) (/), 3% N6AM1400 (0.45g) with quartz (&), sum of 100% F6AM1100 and 3% N6AM1400 with quartz (---). X-axis starts when gases reached the analyzer. Vertical dashed lines represent the ω limit to Fe3O4 and Ni.

3.3. Results from different inert material using methane as fuel

The inert materials investigated can be divided into several sub-groups; Al2O3 based material, stabilized and ordinary ZrO2, TiO2, SiO2 and MgO. In the following sections these groups will be described.

3.3.1. Al2O3 based inert material

This is the largest group of inert material tested. Al2O3 itself has been tested with iron, with and without the addition of the clays bentonite and kaolin. The clays were added to see if the strength of the particles could be increased. Al2O3 and iron do not react during heat treatment in preparation, however a small displacement of X-ray diffraction peaks of Fe2O3 and Al2O3 was noted which indicate some mutual solubility of both metal ions.[190] The addition of bentonite and kaolin to iron oxides with Al2O3 increased the strength of the carriers slightly. However, the reactivity decreased. For the manganese based oxygen carriers mixing with any Al2O3 based inert was avoided due to unsatisfactory results presented by other authors.[137, 143, 146] The combination of copper and alumina was not either tested in this work.

Nickel oxide mixed with Al2O3 results in the formation of NiAl2O4 during heat treatment at about 1000°C. Since this was known in advance, excess NiO was added so that the resulting amount of free NiO would be 40 wt% (as in particles #71-82). NiAl2O4 can be reduced in CH4 but since the reactivity is much slower than for NiO, it is to be seen as an inert material. This has been described in more detail in Paper IV. In general the Ni-based oxygen carriers are much softer in comparison to iron and manganese particles. However, there is a clear increase in strength as a function of temperature, as can be seen from Table 3-4. Thus, in order to achieve acceptable strength, very high sintering temperatures need to be used, up to 1600°C. In order to try and obtain harder particles at lower sintering temperatures, the use of additives to the particles was investigated. Thus, small amounts of kaolin, bentonite and CaO were tested. The nickel oxides with NiAl2O4 and addition of kaolin had lower reactivity and similar strength as those without kaolin. Addition of bentonite maintained the high reactivity whereas the strength became unacceptably low. For addition of CaO the reverse was achieved, high strength but very low reactivity. The use of a different type of Al2O3 starting material was also investigated with NiO.

Particles #71-75 were prepared using a fine and pure α-Al2O3 whereas particle #76 was prepared with a technical grade α-Al2O3 of somewhat higher particles size. It was found that, when compared at the same sintering temperature, the nickel oxide with the pure and fine α-Al2O3 was

62 both harder and more reactive. To summarize, high temperatures need to be employed, above 1500°C, to obtain particles with crushing strength of more than 1 N. This could certainly be a disadvantage when preparing large amounts of material, and it may be of interest to study the effect of other additives and/or preparation procedures in the future.

The last Al2O3 based inert tested was magnesium aluminate, or MgAl2O4. This was tested with both iron and nickel based oxygen carriers. For iron, MgAl2O4 was in general the superior inert, if

60% or less of Fe2O3 was used and the sintering temperatures were below 1150°C. For nickel,

MgAl2O4 was similar or even superior to NiAl2O4 as inert. Mixed with nickel, MgAl2O4 has the advantage of creating both hard and very reactive particles at high sintering temperatures. However, in contrast to the majority of particles manufactured, a sintering temperature of 1100°C, or lower, resulted in both softer and less reactive particles compared to higher temperatures. The reason for this is not clear. An advantage with using NiO/MgAl2O4 as compared to NiO/NiAl2O4 is that the outlet of methane at high degrees of conversion is considerably reduced. On the other hand, there is a drastic decrease in reactivity of these oxygen carriers at lower reaction temperatures. For a more detailed discussion of the difference between

NiAl2O4 and MgAl2O4 as support material to NiO, see Paper IV and XI.

3.3.2. ZrO2 based inert material

ZrO2 has been tested as inert material for all four types of oxygen carriers. Additionally, stabilized zirconia has also been tested on manganese-based oxygen-carriers. ZrO2 has three different structures, monoclinic at < 1170ºC, tetragonal between 1170 and 2370ºC and cubic at temperatures higher than 2370ºC.[191] In the phase transformation from monoclinic to tetragonal, zirconia undergoes a volume change of 3-5%, which can produce cracks in the structure.[192] These types of cracks were probably responsible for the disintegration of iron oxides on zirconia sintered at temperatures over 1100 °C (Paper III). In order to avoid this, zirconia is normally stabilized by any of the oxides CaO, MgO, CeO2 or Y2O3 which dissolves into the zirconia structure and slows down or prevents phase transformations. With this technique the tetragonal and cubic phases can be maintained at room temperature and no phase transformations should occur when passing different temperature zones. The three stabilizing agents used here on manganese based oxygen carriers were CaO, MgO and CeO2. Zirconia is stable and does not interact to any large extent with any of the metal oxides tested.

For iron oxides it is clear that a larger amount of zirconia increases the strength, however, at the same time the reactivity decreases. In general, iron oxides with ZrO2 were not better than with

Al2O3, and have the additional disadvantage of being more expensive. For manganese oxides, Paper V deals with zirconia based oxygen carriers. It was found that addition of stabilizing material greatly increases the reactivity for particles sintered at temperatures of 1150 °C or below in comparison to unstabilized particles. Especially Mg-ZrO2 seemed promising as an inert. The disadvantage could be that some particles tended to de-fluidize, which already has been mentioned in section 3.2.2. For copper oxides the particle mixed with zirconia and sintered at low temperature was the only particle that did not de-fluidize. For nickel oxides the reactivity was very high with zirconia as inert, but the strength was rather low. Using an even higher sintering temperature could possibly result in particles that are competitive with nickel oxides on Al2O3 - based inert material. It could also be speculated that the use of stabilized zirconia could be a good choice for iron- and nickel oxides, although this has not been investigated here.

3.3.3. TiO2 as inert material

TiO2 as inert has been tested on iron, copper and nickel oxides. It has not been used together with manganese based oxygen carriers due to unfavorable results in previous studies.[143] Fe2O3 with TiO2 had very low reactivity and limited crushing strength. Copper particles with titania either de-fluidized or created a hard cake during heat treatment in preparation. Nickel oxides with

TiO2 resulted in rather hard particles, but these were very apt to de-fluidize. Furthermore, the reactivity for the particles sintered at 1100°C or below had a very peculiar behavior with CH4; the reactivity actually increased as the amount of available oxygen in the particles decreased. This can be seen in Paper IV. In general, the results in this work do not give any indications that TiO2 could be a promising inert material for chemical-looping combustion, although Adanéz and co- workers obtained promising results for TiO2 with NiO and CuO.[63, 143, 154, 155, 163]

3.3.4. SiO2 and MgO as inert material

The use of SiO2 and MgO as inert material has not been successful in this work. SiO2 has been tested with iron- manganese- and copper oxide, with poor results, partly due to formation of silicates as described above. Zafar et al. found considerable deactivation of oxygen carriers based on manganese and iron oxides together with SiO2. Better results were found using NiO and CuO together with SiO2, although major deactivation of the Ni-based carrier was found at higher

64 temperatures.[88] Also Adanéz and co-workers reported good results with CuO and SiO2.[63,

143] MgO was only tested with nickel oxide and formation of MgNiO2 was probably the reason for the weak and unreactive particles, as described above.

3.4. Comparison of the oxygen carriers

The rate index as a function of the crushing strength for all particles investigated is presented in Figure 3-8. In this figure the axes are linear simply to illustrate the difficulties in preparing an oxygen carrier with a combination of high mechanical strength and high reactivity. Most particles are either reactive or hard, because a particle with high porosity often has low strength but large area for reaction. Thus there is a trade-off in the choice of sintering temperature, since most particles sintered at low temperatures may be too soft, whereas particles sintered at high temperatures have low reactivity. The notable exceptions are NiO on MgAl2O4 sintered at higher temperatures (#86-88) and NiO on NiAl2O4 sintered at 1600°C (#74-75). Hence, it seems possible to use high sintering temperature to strengthen nickel oxides without too much loss in reactivity.

Figure 3-8 Rate index vs. crushing strength for all investigated oxygen carriers. Numbers are from Table 1-6 to Table 1-9. Circle around number indicates de-fluidization.

To better illustrate the differences in reactivity of the oxygen carriers, the results for all particles are presented on logarithmic scales in Figure 3-9. From the rate index the needed solids inventory

65 in the fuel reactor (kg/MWCH4) was estimated, see Paper III for details and assumptions concerning this calculation. This information is also included in Figure 3-9. The bed masses shown in Figure 3-9 are only indicative and different results can be expected using detailed kinetics and, even more important, a detailed model for the fluidized-bed reactor. No similar calculations on the solids inventory of the air-reactor have been performed; however, a smaller solids inventory than in the fuel reactor is expected. It is clearly seen that there is a large difference in needed solids inventory for the most reactive nickel oxygen carriers compared to the ones based on iron and manganese. A low solid solids inventory would result in a smaller reactor needed, which lowers the capital costs of a combustor. If a solids inventory of less than

500 kg/MWfuel is assumed to be acceptable [37], a majority out of the tested oxygen carriers would be appropriate for chemical-looping combustion.

Figure 3-9 Rate index vs. crushing strength for all investigated oxygen carriers. Numbers are from Table 1-6 to Table 1-9. Circle around number indicates de-fluidization. For comparison, a simple estimation of the solids inventory needed in the fuel reactor is included on the right hand side y-axis. 66

It is not an easy task to generalize the results of the screening investigation carried out in this thesis when using methane as a fuel. However, the main advantages and disadvantages with the four investigated active metal oxides are given in Table 3-6.

Table 3-6 General conclusions on advantages and disadvantages of the four types of metal oxides for use in CLC. MeO + -

Fe2O3 Cheap, Low sintering temperature needed Lower reactivity

Mn3O4 Sufficient reactivity De-fluidization? High reactivity, exothermic reaction with CuO Low melting point, relatively expensive CH4, large oxygen ratio Thermodynamic limitations, toxicity, NiO Very high reactivity, large oxygen ratio expensive

3.5. Results from experiments using syngas as fuel

Because of the vast resources of coal worldwide, it is of interest to adapt chemical-looping combustion for solid fuels. One way to do this is to first gasify the solid fuel to a syngas which contains mainly CO and H2 which can be used as fuel to CLC systems, see section 1.4.3. Three promising oxygen carriers based on iron (particle #12), manganese (#57) and nickel (#86) were chosen for a study in Paper IX. Experiments with 4 and 15 g beds (46 and 173 kg/MWsyngas) were performed at four different temperatures between 650 and 950°C. The composition of the syngas used was 50% each of CO and H2. Note that the definition of γCO here includes possible formation of CH4 from methanation (CO+3H2 ↔ CH4 + H2O). Hence, the partial pressure of methane is also included in the denominator as compared to eq. (12).

Two major conclusions could be drawn from this study. The first is highlighted in Figure 3-10 where the gas yields from experiments with syngas are compared with experiments with methane for the same type of oxygen carriers. Apart from with nickel, where the reactivity is very high and limited by the fuel flow for both systems, it is quite evident that the iron- and manganese-based oxygen carrier have a much higher reactivity towards syngas than methane.

a) b) Figure 3-10. a) The gas yield for CO, γCO, as a function of ω using syngas as fuel at 950ºC and b) the gas yield of methane to carbon dioxide, γCH4, for experiments conducted with 100% CH4 at 950ºC. Ni ( ), Mn (&) and Fe (,) at. Solid lines indicate experiments with 4 g oxygen carriers (in quartz) and dashed lines are the experiments with 15 g oxygen carrier.

In this investigation it was also found that the reactivity of this nickel-based oxygen carrier falls dramatically when the reaction temperature is decreased, whereas the reactivity of Mn and Fe is high and independent of the reaction temperature, see Figure 3-11. The lower reactivity of nickel at decreased temperatures is associated with a quite extensive formation of methane. For manganese, some methane formation was seen at the lower temperatures and at low degrees of conversion of the oxygen carrier whereas no methanation was found for iron oxides. The high reactivity of iron-and manganese oxides with syngas as compared to methane make these oxygen carriers more interesting for CLC with solid fuels.

a) b) c) Figure 3-11 The gas yield, γCO, for a) Ni, b) Mn and c) Fe as a function of ω for the second reduction period at different temperatures: 950°C ( ), 850°C (&), 750°C (*) and 650°C (,). Solid lines indicate experiments with 4 g oxygen carriers (in quartz) and dashed lines are experiments with 15 g oxygen carrier.

3.6. Results from pulse experiments using methane as fuel

Paper XI deals with the use of pulse experiments. In the testing of this experimental technique, two promising nickel-based oxygen carriers which have shown good potential both with respect to CLC and CLR were chosen; N6AM1400 (#86) and N4AN1600 (#74), c.f. Figure 3-9. Examples of pulse cycles can be seen in section 2.4.1. From these experiments information complementary to that of continuous experiments can be obtained. The pulse experiments are especially of interest for CLR(a) because the onset of carbon formation can easily be detected.

This can be seen as a “tail” of CO2 and/or CO leaving the reactor after a pulse and is clearly seen for N6AM1400 in Figure 3-12b. The tail is believed to arise from carbon being oxidized by oxygen still available in the oxygen carriers.

a) b)

Figure 3-12 Concentration profiles from selected pulses of 6 s for N6AM1400 (20 pulses) of a) CO2, b) CO # 1 ("), #8 ((), #9 (*), #13 (,), #15 (-), #17 (/), #19 (4) and #20 (5). Note that the scales on the y-axis differs

By comparing the concentration profiles of carbon-based gases out for each pulse, it is possible to determine if there is carbon formation. When there is no carbon formation, a curve similar to a normal distribution curve appears for the pulses without net carbon formation. However, in later pulses, when tailing of CO2 and/or CO occurs, a deviation at the right-hand side of these curves can be seen, indicating that there has been carbon deposited on the particles. From this deviation the amount of carbon formed can be calculated. The ratio of deposited carbon to carbon added with the methane as a function of pulse number can be seen in Figure 3-13 for the two nickel oxides investigated.

a) b) Figure 3-13 The ratio of deposited carbon to carbon added with the methane for each pulse. a) N6AM1400 and b) N4AN1600

To investigate the oxygen carriers’ suitability for CLR (a), the stoichiometric ratio, λf, as defined in eq. (14) and the outlet methane fraction for these two particles is displayed in Figure 3-14. For comparison the results from continuous experiments are also included in the figure. For an optimized CLR(a) process, λF equals approximately 0.35 and the methane conversion should be complete.[58] In the figures, the two vertical dashed lines indicate in-between which pulses (and its corresponding degree of conversion) that carbon formation was initiated. For N6AM1400 carbon formation was initiated somewhat before such low stoichiometric ratio was reached. But for N4AN1600 carbon formation is initiated already at very high values of the stoichiometric ratio, almost when there still is complete combustion. Even though neither of the particles managed to reach a λF of 0.35 without carbon formation, N6AM1400 is still seen as promising for CLR(a). Carbon formation could, for example, be further suppressed by raising the reaction temperature.

a) b) Figure 3-14 The stoichiometric ratio (pulse&, continuous,) of the gases leaving the reactor and the fraction of methane out (pulse$, continuous+) as a function of the degree of conversion for a) N6AM1400 and b) N4AN1600. In-between dashed vertical lines indicates region where carbon deposition is initiated for the pulse experiments

From the comparison of the two investigated oxygen carriers, N6AM1400 and N4AN1600, in Paper XI, it appears that the former is both more suitable for chemical-looping combustion and the two types of chemical-looping reforming at 950°C. It has a better methane conversion, better reforming properties and is better at avoiding carbon formation. The reason for the latter might be the addition of MgO, since it is can be used as an additive to catalysts in steam reforming to promote gasification of carbon by aiding H2O adsorption.[193, 194]

For N4AN1600, carbon formation is initiated very early in the reduction even though the selectivity to CO2 is very high. Conversely, for N6AM1400 carbon formation takes place when the reaction products are mainly CO and H2.

To conclude, there are two advantages of using pulse experiments in testing of oxygen carriers for use in CLR(a);

• As lower degrees of conversion of the solids are used in CLR (a), the particles that enter the fuel reactor are not fully oxidized as in CLC. However, the particles are expected to be free from any deposited carbon. Since it has been shown that autocatalytic carbon formation has an effect on the methane conversion, at least for some nickel-based

oxygen-carriers, continuous reduction from fully oxidized samples will not give a proper methane conversion at lower degrees of conversion. With pulse experiments, every pulse starts without any deposited carbon on the particles, which is a more realistic situation. • With pulse experiments small snapshots of the reaction products can easily be obtained.

By combining these snapshots with investigation of possible tailing of CO2 and CO, start and extent of carbon formation is obtained.

3.7. De-fluidization

As can be seen in Figure 3-9, a large fraction of the investigated oxygen carriers de-fluidized at some point during the experiments. The occurrence of de-fluidization and in some cases agglomeration is hard to predict and understand. Cho et al investigated this phenomenon for iron-, nickel- and one manganese-based oxygen-carrier(s) and concluded that agglomeration of iron based particles was clearly associated with the formation of wustite (FeO).[162] This generally suggests that de-fluidization of iron-based oxygen carriers should not be a problem in a real CLC application because reduction to wustite is irreconcilable with a high yield of CO2 and

H2O.[90, 91] For the nickel-based oxygen-carriers, a clear conclusion could not be made and the manganese-based oxygen-carrier investigated did not de-fluidize at all.[162]

De-fluidization in this work does not necessarily mean that a hard agglomerate was formed in the bed, in most cases only a small force was necessary to separate the particles from each other. In general, among the particles that de-fluidized during experiments, all iron- and copper oxides as well as the nickel-oxides with titania as inert tended to form hard agglomerates whereas the other nickel oxides and all manganese oxides formed very soft agglomerated structures. However, it is believed that the de-fluidizations that have occurred in experiments conducted in this work are mainly associated with different physical aspects of the reactor and other experimental conditions. Hence, under different operational conditions, for instance with higher velocities in the bed, it is believed that de-fluidization for most of the oxygen-carriers could be avoided.

a) b)

c) d)

Figure 3-15 Crushing strength as a function of apparent density for a) Iron-based oxygen-carriers, b) Manganese-based c) Copper-based and d) Nickel-based. Circle around number indicates de-fluidization. Numbers are from Table 1-6 to Table 1-9

The crushing strength as a function of the apparent density for all oxygen carriers investigated is presented in Figure 3-15. From this figure it is seen that the crushing strength of the particles is proportional to the apparent density (except for copper-based particles). Since de-fluidization is also presented in this figure (as circles), another correlation is highlighted for the iron- and, to some extent nickel-based oxygen-carriers; de-fluidization is mainly associated with high density particles, above 2500 kg/m3. For the manganese based oxygen carriers no clear correlation is found. For copper oxides de-fluidization is mainly believed to be an effect of the rather low melting temperature of Cu0. This can also be seen from SEM images of copper-based oxygen- carriers where a melting of the surface seems to have taken place. Even though the experiments 73 are performed at lower temperatures than the melting temperature, local sintering could occur. It has been suggested that sintering could take place when exceeding the Tammann temperature, which is 0.4-0.5*melting temperature (K).[195, 196]. Above this temperature, metal crystallites could get liquid-like properties which could enhance their ability to migrate, especially in the presence of hydrogen.[196] Hence, for all oxygen carriers, in all experiments, the Tammann temperature is exceeded, but copper-oxides exceed their Tammann temperature relatively more which may make them more vulnerable for agglomeration.

A possible reason for the fact that de-fluidization mainly concerns particles of high density, at least for iron- and nickel-oxides, may be correlated to the relative fluidizing velocities. Since the flow is equal for all experiments because of the use of a standardized test procedure, the ratio of velocity to minimum fluidizing velocity will be lower for the high density particles, which is a disadvantage from a fluidizing point of view. Another possible reason for the de-fluidization could be due to the bed height. The diameter of the reactor is 30 mm and the bed height between 4-33 mm, depending on the density of the particles. Hence, de-fluidization could also be associated with channel formation, which could be more likely to take place in a shallow bed, i.e. when using particles of large density. These two reasons may explain why 15 g M4MgZ1150 de- fluidized in the 30 mm reactor used here but not in the 22 mm reactor as used by Cho et al.[162] There are also differences in the degree of conversion in the experiments, which could have an effect. This could be the case for the manganese-based oxygen-carriers, as de-fluidization seems more associated with the reactive particles, c.f. Figure 3-3. This is because the reduction was terminated before the carrier was fully reduced for some of the particles with lower reactivity, see Paper V.

In association with the discussion above, a number of freeze-granulated particles manufactured in larger batches and tested in the 300 W and 10 kW chemical-looping combustors have been run in operation for extended periods without any signs of de-fluidization.[73, 74, 82-85] An interesting exception is the formation of smaller agglomerates of N6AM1400 as noted by Rydén et al.[86] These particles were not used for CLC but for CLR(a) where conditions are more severe, with carbon formation and more reduced particles. In these 300 W and 10 kW experiments the velocities are higher, the beds are much higher and a smaller conversion interval, ΔX, is used. Generally it should be pointed out that de-fluidization/agglomeration could be expected to be less likely at the higher velocities in full-scale units. The velocities in these laboratory tests are around 0.1 m/s compared to expected velocities in a full-scale riser of around 5 m/s.

3.8. Carbon Formation

For some of the manganese-, copper-, and nickel- oxides investigated in this thesis, carbon formation has occurred. This means that carbon has formed on the particles, most likely through either of the two reaction mechanisms of methane pyrolysis (15), or the Boudouard reaction:

2CO → CO2 + C (17)

Both reaction (15) and (17) are catalyzed by Ni0 (and most likely also Cu0) whereas manganese oxides are not known to be catalytically active in these reactions. The carbon formation in the experiments was detected as CO2 and/or CO leaving the reactor in the inert and/or oxidation period. In the oxidation this was noticed either directly when O2 is introduced into the reactor

(for copper- and nickel-carriers) or when O2 is starting to pass unreacted through the bed of oxygen carriers (manganese-carriers). The difference between these behaviors could explain where the deposited carbon is located in the reactor; for copper- and nickel-carriers the carbon is in connection with the bed material and for the manganese carriers the carbon is above the bed, e.g. at the exit of the reactor. For the manganese-based oxygen carriers this fact is further verified by the observation of carbon at the top of the reactor, when terminated after a reduction period, and that carbon was not found in the XRD analyses for any of the reduced beds. Examples of release of CO2/CO in the oxidation period can be found in Papers IV and V.

Based on thermodynamical equilibrium, carbon formation should not be a problem at the conditions used in a CLC reactor system where conversion of the fuel is high. Thus, at a temperature of 950°C no carbon formation is expected as long as more than one fourth of the oxygen needed for complete fuel conversion is supplied.[90, 91] In laboratory experiments, however, the reduction is often allowed to proceed until the conversion of the gas is low. Cho et al found that major carbon formation in laboratory experiments was correlated to low conversion of the fuel.[153] However, in both Paper VIII and XI carbon formation was noticed for N4AN1600 even at high conversion of the fuel. As in the case of de-fluidization, carbon formation seems to be associated with the laboratory fluidized-bed experiments and is not seen in the 300 W and 10 kW chemical-looping combustors. [73, 74, 82-85] Rydén et al. found carbon formation with N6AM1400 for CLR(a) but that was clearly associated with the very low degrees of oxidation of both fuel and the oxygen carriers used.[86]

3.9. Kinetics

Kinetics of oxygen carriers have been investigated in some studies.[36, 64, 115, 126, 132, 134, 160, 165, 172, 175, 176, 186, 187] In these studies, nickel-, copper-, manganese- and iron oxides of different particle diameter, amount of active oxygen and manufacturing methods have been tested. From these it can be concluded that the reduction reaction is controlled by chemical reaction resistance and the oxidation by either only chemical reaction resistance or both chemical reaction resistance and product layer diffusion.

In some of these studies, the order of the reaction for reduction of oxygen carrier with methane has also been investigated. [64, 132, 172, 175, 186, 187] The values obtained vary between 0.4 and 1.3.

The rate index was introduced in the papers in this thesis as a way to give a number to the reactivity in order to facilitate comparisons, e.g. Figure 3-9, based on the testing procedure developed. The rate index is clearly a simplified way of describing reactivity, and cannot replace a detailed kinetic study.

The rate index calculated and explained in section 2.4 in this thesis is a normalized average reaction rate based on the following assumptions:

• First order of reaction • No mass transfer resistance between bubble and emulsion phases • Average conversion rate was calculated as an average for an interval of 1% of mass reduction, Δω, when the rate was highest

• Conversion rate was normalized with a mean partial pressure of CH4 of 0.15 over the bed. This corresponds to very high conversion of the methane

Hence, the first assumption, a first order of reaction may not be valid in view of most of the results obtained by the above mentioned work. Instead it is shown that the order of reaction, n, often is lower than 1. To see what effect a lower order of reaction has on the assumptions on solid mass inventory shown in section 3.4, an illustrative example will follow below:

In order to illustrate the effect of the reaction order we assume that we have a gas that reacts to another gas as it passes over a bed of reactive material. The partial pressure will then decrease and the decrease is dependant on the amount of reactive material, m the partial pressure, p, and the reaction order, n. This can be described as follows when the gas passes a small segment of bed material, dm:

dp = −k ⋅ pn (18) dm where k is a constant which reflects the reactivity of the material. If eq. (18) is integrated, the outlet partial pressure, p2, for a bed with a mass of m is obtained:

−km p2 = p1e , n = 1 (19a)

(1−n) 1/(1−n) p2 = ()p1 − km(1− n) , 0 < n < 1 (19b)

Here, p1 is the ingoing partial pressure. By rearranging eqns (19a) and (19b) the constant can be obtained as function of the mass and in and outgoing partial pressures.

⎛ p2 ⎞ − ln⎜ ⎟ ⎝ p1 ⎠ k = , n=1 (20) m

− (p (1−n) − p (1−n) ) k = 2 1 , 0 < n < 1 (21) (1− n) ⋅ m

Thus, the constant k can be obtained from an actual experiment. With eqns. (19a) and (19b) the outgoing concentration can then be obtained as function of the mass of bed, m.

st th If we assume an experiment was made with m=1, p1=1 and p2=0.2, using a 1 and 0.5 order of reaction it will give two different results according to equations (19), (20) and (21). In Figure

3-16, p2 is shown as function of bed mass, for the two cases. As seen in the figure, the two graphs intersect at a mass of 1 and an outgoing partial pressure of 0.2 as were the given input data. When calculating the rate index, the reaction rate is normalized to a reference outgoing partial

77 pressure of CH4 of 0.001. Normally, such low partial pressures are not obtained in the laboratory fluidized-bed experiments. Hence the values of the needed bed mass are extrapolated to a lower outgoing partial pressure of CH4, which is illustrated in Figure 3-16b. For simplicity, 0.01 is used

st in the figure. If a 1 order of reaction is valid, the mass increase needed corresponds to Δm,2 in the figure and for a 0.5th order of reaction the corresponding mass increase is represented by

Δm,1. In percent, these examples correspond to an increase of bed mass with 286 and 163% respectively.

a) b)

Figure 3-16 Examples of outgoing partial pressure of methane as a function of mass for two different values of n: the order of reaction. n=1 (—), n=0.5 (– –). a) Full interval, b) partial pressure interval of 0.2 and below

The conclusion of this example is that if the calculations are based on a 1st order of reaction, but the real order of reaction is lower, the predictions used in this thesis result in an overestimation of the needed mass. However, if the outlet pressure of CH4 in an experiment is below the outlet reference pressure of 0.001, the opposite would be valid, i.e. a first order of reaction would underpredict the solids inventory. Such high gas conversions are, however, too small to be measured. Thus it can be concluded that the first order assumption is conservative, i.e. underestimates the needed mass.

4. Conclusions

When the work in this project was initiated, only a limited number of oxygen carriers had been investigated for CLC using methane as fuel. This work presents a comprehensive screening including 108 different types of oxygen carriers based on iron, manganese, copper and nickel. A testing methodology was established in order to investigate the oxygen carrier particles with respect to parameters important for chemical-looping combustion, including reactivity under alternating oxidizing and reducing conditions, de-fluidization and agglomeration, crushing strength as well as chemical and physical characterization. For some promising oxygen carriers, additional investigations were performed in order to increase the understanding of the system. Some oxygen carriers were also investigated for use with syngas from coal gasification and hydrogen production through CLR(a). Of the investigated oxygen carriers, several were found to have properties suitable for CLC, and five of the oxygen carriers have been produced in larger batches and their performance was later verified in 10 kW and 300 W chemical-looping combustors.[73, 74, 82-86] The main conclusions from the study are:

• Iron-, manganese- and copper-oxides have similar reaction behavior in the reduction

where all methane initially reacts with the oxygen carriers to CO2 and H2O. When the amount of available oxygen decreases, unreacted methane will pass through the bed and

small amounts of CO and H2 can be seen. For nickel oxides there is also almost full

conversion to CO2 and H2O initially, but unreacted methane will not pass through the 0 reactor as it is instead reformed to CO and H2 by Ni .

• Nickel-based oxygen-carriers produce the most reactive oxygen carriers but cannot

convert the fuel completely to CO2 and H2O. The other three oxides roughly follow the order Cu>Mn>Fe with respect to reactivity towards methane.

• Sintering temperature as the last step in manufacturing can be used to optimize the strength-porosity characteristics. Usually a higher sintering temperature gives stronger but less reactive particles. For iron- and manganese-based particles, sintering temperatures between 950 and 1150°C were optimal, whereas for the nickel based oxygen carriers the appropriate range was 1300°C and upwards. Iron oxide particles generally have the highest strength.

• Iron- and manganese oxides have a much higher reactivity towards syngas than methane. A way of taking advantage of this fact, when using methane as fuel, is to use mixed oxides. This means mixing small amounts of nickel-oxides to a bed of either iron- or manganese oxides, thereby using nickel to catalyze methane conversion to a syngas mixture which increases the overall reaction rate. In this thesis, this has been demonstrated using a mixed bed of nickel- and iron-based oxygen carriers.

• Among the different inert material tested, the Al2O3 and ZrO2 groups have shown most

promising results. MgAl2O4 was the best inert tested for iron oxides followed by Al2O3.

For nickel oxides both NiAl2O4 and MgAl2O4 were found to be excellent inert materials.

ZrO2 was also a very promising inert for nickel oxides and different types of stabilized- zirconia were the best inerts found for manganese-based oxygen-carriers. The other inert

materials tested, TiO2, SiO2 and MgO, were less satisfying.

• A majority of the oxygen carriers were reactive enough to give an estimated solids

inventory in the fuel reactor of less than 500 kg/MWCH4.

• Pulse experiments can be used as a complement to continuous experiments. The main advantage with this type of experiments is when investigating particles suitable for syngas production in autothermal chemical-looping reforming, CLR(a). This is because smaller conversion ranges can be studied in detail and the onset of carbon formation at low degrees of solids conversion is easily detected.

• De-fluidization of the beds occurred in many experiments. For iron- and nickel oxides this phenomenon is closely connected to particles of high strength, which means particles of lower porosity, higher apparent density, lower ratio of velocity to minimum fluidizing velocity and lower bed height in the experiments. Thus, different fluidization conditions may contribute to the de-fluidization of these denser particles. However, for the manganese-based oxygen carriers, de-fluidization occurs for the most reactive particles which should have the best fluidization properties for the experiments, i.e. a higher bed due to their lower densities. Copper oxides de-fluidize easily which is believed to be associated with the low melting temperature of Cu0.

5. Notations k = constant (bar1-n g-1) -1 -1 keff = effective reaction constant (bar s ) m = actual mass of oxygen carriers (g) mred = mass of oxygen carriers in reduced form (g) mox = mass of oxygen carriers in oxidized form (g) n = order of reaction (-) p = partial pressure (bar) p1 = ingoing partial pressure (bar) p2 = outgoing partial pressure (bar) pi,in = partial pressure of gas i entering the reactor (bar) pi,out = partial pressure of gas i exiting the reactor before water has been removed (bar) pm = log-mean partial pressure of methane (bar) pref = reference partial pressure of methane, chosen to 0.15 (bar) Rate index = normalized reaction rate (%/min)

RO = oxygen ratio (-) t = time (s) X = Conversion, or degree of oxidation of the oxygen carrier (-)

γf===gas yield of gaseous specie i (-)

λF = stochiometric ratio (-)= ω = mass based conversion, or degree of oxidation of the oxygen carrier (-)

6. Acknowledgement

I would like to thank the following people for helping me with the work for this thesis or by making my spare time as enjoyable as possible:

My main supervisor Associate Professor Tobias Mattisson and supervisor Professor Anders Lyngfelt for their support and for all valuable discussions we’ve had through the years. I thank you for letting me work with CLC in the first place but also for being very kind and full of encouragement.

Professor Oliver Lindqvist for providing me with the opportunity to work at the Department of Environmental Inorganic Chemistry at Chalmers University of Technology.

The former and current nice people of the CLC group at Chalmers for various help and valuable discussions. Thank you Paul, Eva, Qamar, Magnus, Alberto, Henrik, Erik, Calle and Nicolas.

Esa, Roger and Charlotte for helping me with various stuff way more difficult than CLC.

I am forever grateful to the people of CSIC in Zaragoza for giving me the basic knowledge of CLC and for the good time I had in Spain during my Diploma thesis.

Also a great thanks to the financers of the studies; the ECSC project, Capture of CO2 in Coal Combustion (CCCC), 7220-PR-125, the EU financed research project Grangemouth Advanced

CO2 Capture Project (GRACE), ENK5-CT-2001-00571, led by BP and CO2 Capture Project (CCP), EU project Cachet, Contract Nr 019972 (SES6) and the Swedish Energy Agency, projects 20274-1 and 21670-1.

To Spain, Mexico, La liga, books and music for utterly important recreation time.

And last but definitely not least; a great thanks to my wonderful family and great friends for all support and good times!

7. Appendix – XRD of investigated oxygen carriers

Fresh After red. After ox. Comment F4S950 Fe2O3, SiO2 - - F4S1100 Fe2O3, SiO2 - - F4S1300 Fe2O3, SiO2 (2) - - F4Z950 Fe2O3, ZrO2 Fe3O4, ZrO2 - F4Z1100 Fe2O3, ZrO2 Fe3O4, ZrO2 - F4Z1300 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2, ZrSiO4 - F6Z1200 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2 - F6Z1300 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2, ZrSiO4 - F6Z1400 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2, ZrSiO4 - F8Z950 Fe2O3, ZrO2 Fe3O4, ZrO2 - F8Z1100 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2, ZrSiO4 - F8Z1300 Fe2O3, ZrO2, ZrSiO4 Fe3O4, ZrO2, ZrSiO4 - F22A1100 Fe2O3, Al2O3 FeAl2O4,Al2O3 - F6A950 Fe2O3, Al2O3 Fe2O3, Al2O3, FeAl2O4 - F6A1100 Fe2O3, Al2O3 Fe2O3, Al2O3, FeAl2O4 - F6A1300 AlFeO3 Fe2O3, Al2O3, FeAl2O4 Fe2O3, Al2O3 F6A1300 Fe2O3, Al2O3 Fe3O4, Al2O3 Fe2O3, Al2O3 F6AB1100 Fe2O3, Al2O3 Fe3O4, Al2O3 Fe2O3, Al2O3 F6AB1200 Fe2O3, Al2O3 Fe3O4, Al2O3 - F6AB1300 Fe2O3, Al2O3 Fe3O4, Al2O3 - F6AK950 Fe2O3, Al2O3 Fe2O3, Al2O3, FeAl2O4 - F6AK1100 Fe2O3, Al2O3 Fe2O3, Al2O3, FeAl2O4 - F6AK1300 AlFeO3 Fe2O3, Al2O3, FeAl2O4 - F4AM1100 Fe2O3, Mg(Al,Fe)2O4 Mg(Al,Fe)2O4 - Uncertain F4AM1200 Mg(Al,Fe)2O4 Mg(Al,Fe)2O4 - Uncertain F4AM1300 Mg(Al,Fe)2O4 Mg(Al,Fe)2O4 - Uncertain F6AM950 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 - Uncertain F6AM1100 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 Fe2O3, Mg(Al,Fe)2O4 Uncertain F6AM1125 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 - Uncertain F6AM1150 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 - Uncertain F6AM1175 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 - Uncertain F6AM1200 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 Fe2O3, Mg(Al,Fe)2O4 Uncertain F6AM1300 Fe2O3, Mg(Al,Fe)2O4 MgFeAlO4, Mg(Al,Fe)2O4 - Uncertain F8AM1100 Fe2O3, Mg(Al,Fe)2O4 MgFe3O4, MgFeAlO4 - Uncertain F8AM1200 Fe2O3, Mg(Al,Fe)2O4 MgFe3O4, MgFeAlO4 - Uncertain F8AM1300 Fe2O3, Mg(Al,Fe)2O4 MgFe3O4, MgFeAlO4 - Uncertain F4T950 Fe2TiO5, TiO2 FeTiO3, Fe3Ti3O10, TiO2 - F4T1100 Fe2TiO5, TiO2 FeTiO3, Fe3Ti3O10, TiO2 - F4T1300 Fe2TiO5, TiO2 FeTiO3, TiO2 Fe2TiO5, TiO2 F6T950 Fe2TiO5, TiO2 FeTiO3, Fe3Ti3O10 - F6T1100 Fe2TiO5, TiO2 FeTiO3, Fe3Ti3O10 - F6T1300 Fe2TiO5, TiO2 FeTiO3, Fe3O4 - M37S950 Mn7SiO12, SiO2 - - Uncertain M37S1100 Mn7SiO12, SiO2 etc. Mn2SiO4, SiO2 etc. - Uncertain M37S1200 MnSiO3, SiO2 (3) MnSiO3, SiO2 (3) - Uncertain M37S1300 Mn7SiO12, MnSiO3, SiO2 etc. Mn2SiO4, MnSiO3, SiO2 etc. - Uncertain M37Z950 Mn3O4, ZrO2 MnO, ZrO2 - M37Z1100 Mn3O4, ZrO2 MnO, ZrO2 Mn3O4, ZrO2 M37Z1300 Mn3O4, ZrO2 MnO, ZrO2 Mn3O4, ZrO2 M37Z950 Mn3O4, ZrO2, C MnO, ZrO2 - M37Z1100 Mn3O4, ZrO2, C MnO, ZrO2 - M37Z1300 Mn3O4, ZrO2 (2) MnO, ZrO2 (2) - M6Z1200 Mn3O4, ZrO2 MnO, ZrO2 - M6Z1300 Mn3O4, ZrO2 MnO, ZrO2 - M78Z950 Mn3O4, ZrO2 MnO, ZrO2 - M78Z1100 Mn3O4, ZrO2 MnO, ZrO2 - Mn3O4, ZrO2, Mn3O4, ZrO2, M4CaZ950 MnO, ZrO2, Ca0.15Zr0.85O1.85 Ca0.15Zr0.85O1.85 Ca0.15Zr0.85O1.85

Mn3O4, ZrO2, M4CaZ1100 MnO, ZrO2, Ca0.15Zr0.85O1.85 - Ca0.15Zr0.85O1.85 Mn3O4, ZrO2, M4CaZ1200 MnO, ZrO2, Ca0.15Zr0.85O1.85 - Ca0.15Zr0.85O1.85 M4CaZ1300 Mn3O4, ZrO2 MnO, ZrO2 - M4MZ950 Mn3O4, ZrO2 MnO, ZrO2 - M4MZ1100 Mn3O4, ZrO2 MnO, ZrO2 - M4MZ1150 Mn3O4, ZrO2 MnO, ZrO2 - M4MZ1200 Mn3O4, ZrO2 MnO, ZrO2 - M4MZ1300 Mn3O4, ZrO2 MnO, ZrO2 - M4CeZ950 Mn3O4, ZrO2, Zr0.84Ce0.16O2 MnO, ZrO2, Zr0.84Ce0.16O2 - M4CeZ1100 Mn3O4, ZrO2, Zr0.84Ce0.16O2 MnO, ZrO2, Zr0.84Ce0.16O2 - M4CeZ1300 Mn3O4, ZrO2, Zr0.84Ce0.16O2 MnO, ZrO2, Zr0.84Ce0.16O2 - M26M950 - - - MgO, Mg0.9Mn0.1O, M26M1100 Mn3O4, MgO, Mg6MnO8 - (MgO)05.9(MnO)0.41 C6S1100 CuO, Cu2O, SiO2 Cu, SiO2 - C4T950 CuO, TiO2 Cu, TiO2 - C4T1100 - - - C4Z950 CuO, ZrO2 Cu, ZrO2 - C4Z1100 CuO, Cu2O, ZrO2 Cu, ZrO2 - N4Z950 NiO, ZrO2 Ni, ZrO2 - N4Z1100 NiO, ZrO2 Ni, ZrO2 - N4Z1300 NiO, ZrO2 Ni, ZrO2 NiO, ZrO2 N4AN1300 NiO, NiAl2O4 Ni, NiAl2O4 NiO, NiAl2O4 N4AN1400 NiO, NiAl2O4 Ni, NiAl2O4 - N4AN1500 NiO, NiAl2O4 Ni, NiAl2O4 NiO, NiAl2O4 N4AN1600 NiO, NiAl2O4 Ni, NiAl2O4 NiO, NiAl2O4 N4AN1600 NiO, NiAl2O4 Ni, NiAl2O4 NiO, NiAl2O4 N4AN1600 NiO, NiAl2O4 Ni, NiAl2O4 - N4AK950 NiO, Al2O3 Ni, Al2O3 - N4AK1100 NiO, NiAl2O4 Ni, NiAl2O4 - N4AK1300 NiO, NiAl2O4 Ni, NiAl2O4 - N4AK1400 NiO, NiAl2O4 Ni, NiAl2O4 NiO, NiAl2O4 N4AC1400 NiO, NiAl2O4, CaSiO3 Ni, NiAl2O4, CaSiO3 - N4AC1600 - - - N4AB1300 NiO, NiAl2O4 Ni, NiAl2O4 - NiO, MgAl2O4, N6AM950 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 NiAl2O4 NiO, MgAl2O4, N6AM1100 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 NiAl2O4 N6AM1300 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 - NiO, MgAl2O4, N6AM1400 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 NiAl2O4 N6AM1500 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 - N6AM1600 NiO, MgAl2O4, NiAl2O4 Ni, MgAl2O4, NiAl2O4 - N6M1100 MgNiO2 - - N6M1200 MgNiO2 - - N6M1300 MgNiO2 MgNiO2 - N4T950 NiO, NiTiO3, TiO2 (2) Ni, TiO2 NiO, NiTiO3, TiO2 N4T1100 NiO, NiTiO3, TiO2 Ni, TiO2 - N4T1300 NiTiO3, TiO2 Ni, TiO2 NiO, NiTiO3, TiO2 N8T950 NiTiO3, NiO Ni, TiO2 - N8T1100 NiTiO3, NiO Ni, TiO2 - N8T1300 NiTiO3, NiO - NiTiO3, NiO, Ni Numbers in brackets indicate more than one type of phase

8. Appendix – SEM images of investigated oxygen carriers

Below are displayed SEM images of the surfaces of all investigated oxygen carriers. Both fresh particles and particles tested one day in the laboratory fluidized-bed reactor and saved in reduced state, are shown. The numbers in the margin are taken from Table 1-6 - Table 1-9. (O.C. =oxygen carrier). Horizontal white bar at bottom = 45 μm, c.f. with Figure 2-1c.

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